BLR
12-19-2012, 07:53 AM
I am not divulging any proprietary information given to me, as I have no non disclosure agreement with anyone in the firearms coating industry. Anything contained below is available in the literature for anyone to find themselves. If you are in the industry, and I have been mistaken about your particular system – my apologies, please correct me! Also, the following are (more or less) generic opinions. I am not interested in arguing with anyone about anything (since I’m not paid by this, I have no interest in arguing about it).
There are several reasons to coat metals (like a 1911) – aesthetics, corrosion protection, and wear protection. While the first category is obviously subjective, the second and third are much less so – and are often interwoven. Because aesthetics are subjective, I will only offer that charcoal bluing is the “be all end all” of bluing. Nothing else looks as good. Only rust bluing comes close. Hot blue is ugly. This is opinion, feel free to disagree all you like. Bluing by the way, provides nearly no protection from corrosion or wear. It is basically, in any of its iterations, simply a forced oxide layer growth. Few metals (Cr and Al come to mind) form a real “passivation” layer (corrosion engineers term to describe a naturally stable oxide formation preventing further oxide formation). The iron in steel isn’t one of them. Stainless steel gets its corrosion resistance from a high Cr content, typically 15ish% and up. It’s actually the Cr on the surface that passivates the steel, the stainless steel guys term that “throw.”
Corrosion and Wear Protection. We will only treat the corrosion and wear protection of steel (and in particular carbon steel) – these topics for aluminum and titanium are quite “deep” and fraught with nuances that are difficult to explain fully. That and 1911s are steel. Most of what is said can be transferred to stainless, and to a lesser extent, aluminum. Ti is a whole world in and of itself.
Ok – so we have two issues. One, we don’t want our $3k SRP to wear out. Two – we really don’t want it to ever rust. We have a several of potential routes available to us to remedy the situation. These include metallic coatings (eg – chrome and nickel); polymer coatings (eg – epoxy-PTFE); composite (eg – TiN, as in drill bits and that gawd aweful gold plating some people insist on, or electroless nickel); chemical, really two types – surface (eg – parkerizing, electroless nickel) and penetrating (eg – carborizing/carbonitriding); and other. The “other” is added to include those people who claim to be a ceramic coating or whatever.
The first category (metallic coatings) is perhaps the best and worst at the same time. Chrome is king here. There are other metallic coatings and claddings used, such as nickel. But none are as popular as good old industrial hard chrome. Chrome is almost universally deposited electrochemically in a bath of (among other things) hexavalent chromate. Consequently, that puts the plater up high on the EPA Hate List. There are other ways to do it, but are rarely ever seen (if at all) in firearms. This chemical (Cr2O7) is extremely toxic to people and the environment, and is the reason chrome coatings won’t be around forever. Chrome can be deposited thick enough to close up machine tolerances. When done right, will last forever. When done wrong, it results in cracked parts (like frames and slide stops), blistering, peeling, and sub chrome layer corrosion. This typically happens when the plater gets into a rush and deposits too quickly, resulting in the evolution of hydrogen gas at the surface of the part – leading to hydrogen embrittlement, holidays (defects), and uneven coating thickness. This is typically because of too thick of a coating or poor pre-cleaning. Some engineers (I’m one) argue that there is no way to prevent hydrogen embrittlement as a result of chrome plating. As a consequence, you will typically see a long term post bake (typically 400F or so) in a vacuum for critical parts. This forces the hyrdogen gas in the metal to evaporate from the metal (really cool phenomena actually). One can get two pieces of valuable information from this empirically – 1) chrome plating has its place (if the aerospace industry will tolerate the shortcomings, there is a reason!); and 2) chrome parts failure is common enough there is a procedure to minimize it. Nickel is close to chrome on the periodic table (same row, D block), and it stands to reason that the two metals behave roughly equally. They have roughly the same hardness and coefficient of friction. In fact, Cr, Co, and Ni were all there in the original “super alloys” for turbine engine blades. There are slightly different corrosion properties between the two, but suffice to say they are both better than plain carbon steel. As a gun guy (and an engineer), I really don’t like these coating systems. Too many ways to cause problems, and not that great rate of a return on wear and corrosion prevention. A point of clarification – here I am discussing elemental chrome and nickel coatings. On the plus side (as I see it), this is an ideal way to show off your gunsmithing talents – as chrome doesn’t hide ANYTHING!
The second category (polymer) has the greatest number of snake oil salesmen and BS floating around. One must keep in mind that polymer coatings are typically (and rightfully!!!) used in conjunction with surface chemical treatments. The most common surface treatments are: parkerizing (think Armor Tuff); silaninzing (don’t hear much about this in the firearms industry for some odd reason); electroless nickel; and chromate conversion coatings (again, not too popular, but for better reasons than silanes). These represent the worst in wear resistance, as would be expected. Polymers are just soft compared with metals. There are some who say corrosion protection remains intact after the coating wears through. Unless the coating is an active anti-corrosion coating (such as a chromate conversion coating – CCC), this is simply untrue. What typically happens is the places the coating wears through simply are high wear areas, and through natural abrasion, doesn’t allow the formation of a visible layer of rust. It’s still there, but you can’t see it with the naked eye. Polymer coatings are sometimes (by hacks) used like body filler on a car – to hid mistakes. There seems to be a wide variation in the properties of the polymer coatings out there. This is partially ture – some use different fillers and the properties vary accordingly. But the basic epoxy chemistry, for the most part, is consistent across the board. For example, if Brand X is often cited as a “softer” coating, it likely has more Teflon particles in it resulting in a thicker and softer appearing coating. With polymer coatings, “hardness” and “thickness” are intertwined (at least as far as most shooters are concerned). There is little variation in the actual epoxy resin and hardener in use by most shops. One of the more clever (and better, IMO) variations is a system that system has an underlying electroless nickel (nickel phosphate) deposited before coating. Functionally, polymer coatings offer a good compromise of protection and economics. They are cheap to apply and reapply as compared with metal and composite coatings generally. Cerakote is one of the more distinct polymer coatings out there. The word around the campfire is that this has some ceramic additives to enhance wear performance. I called NIC and they wouldn’t tell me. So I put a sample through our HPLC-MS, and mostly figured out what it really is. This represents a 2-5x increase in wear resistance over simple PTFE filled epoxy. This type (polymer) represents the lower end of both corrosion and wear protection when compared against all the other systems. However, it is also the least expensive and easiest to maintain. These also don’t typically suffer from the shortcomings seen with other coatings, like hydrogen embrittlment, cracking, and difficulty in future gunsmithing.
Category three (thin films, aka hard coatings, aka composite coatings) represents the richest field of tribology in my opinion. The only real contender would be chemical treatments (like carbonitriding) in terms of surface hardness. There is, in my opinion, room for interpretation in the distinction between say carborizing and carbide coating. This is because of the application techniques more than anything. I will try not to stray into chemical modifications (carborizing) tonight. Rather, we will discuss primarily coatings in their more rigorous definition. A couple of observations - these coatings are almost universally ceramics. Which are often used in grinding wheels. So unless you have two smooth surfaces, you have created two abrasive mating surfaces. I trust the implications of that are obvious.
There are many different types of coatings. They are typically iron, aluminum, chromium titanium, boron, and carbon based. For example: iron carbide, aluminum nitride, titanium carbon-nitride, boron carbide, and diamond like carbon. More generically, we are talking about carbides and nitrides (and mixtures of these). There are two general modes of coating applications: Physical Vapor Deposition and Chemical Vapor Deposition. PVD is typically used to produce DLC and thinner films of carbides and nitrides. CVD is typically used to produce thicker films, and lacks the fine control of morphology and composition that PVD typically provides. PVD coatings are typically less than 0.0005in, and CVD coatings are typically less than 0.001in. Both of these numbers are on the HIGH end of the range. They all have one unique thing in common - extremely high hardness. This gives them their most attractive feature - wear resistance. They in fact, were often developed for the tooling industry. And when I say tooling, I mean cutting tools. Like lathe inserts. These tools were developed, among other things, to allow single point cutting of "hard materials". Hardinge (IIRC) coined the phrase "Hard Turning" to describe this process. Prior to the development of these cutting tools, the shaping of nickel/chrome materials was accomplished by almost exclusively grinding operations. This is because the cobalt "tool" steel couldn’t effectively cut these materials. Ironically, this resulted in the development of powder metallurgy, and in particular, Metal Injection Molding, to allow the production of the cutting tools. So next time someone chimes in and preaches the virtues of tool steel and the evils of MIM, remember the cutting tool used to produce his tool steel are often produced via MIM! So these coatings provide (potentially!!!) one piece of the equation - wear resistance. In fact, micro hardness of many PVD/CVD coatings hovers in the 80Rc range, where tool steel is around 60ish Rc (high speed steel is a little higher, 62-64Rc). Great - we now have our wear resistant coating system! Now something that typically isn't discussed by people, which is a frequent problem in firearms, is adhesion. Remember I said PVD allows better composition control - well, CVD typically bonds better. This is because CVD is a "hot surface" process. The part to be coated is heated (to 400-1200C) and a vapor phase reactant is introduced into the chamber. The molecules of the gas injected react on the surface of the hot part, forming the film. A result of the elevated temperature is a "diffusion" bonding of the coating to the part. Remember, there are NO free lunches! So, hardness is universally high for these coatings. However, lubricity (a function of coefficient of friction) is all over the map. It really depends greatly on the individual process.
The other side of our equation is corrosion resistance. Here is the rub of these coatings - if a "holiday" exists, sever corrosion can result in the underlying metal. However, done right, it provides excellent oxidation resistance. Typically, though, these coatings are “line of sight” deposition techniques, so coating the inside of a firing pin channel is difficult.
Some other considerations and thoughts - these are the most expensive to apply. And you will be getting a coating developed for the tooling industry, not the firearms industry. Aesthetically, these range from the beautiful to straight up ugly! They can go from a look similar to stainless/bare steel (CrN for example) to a charcoal black (TiCN) to dark black (DLC) to a gawd aweful gold (TiN). Careful consideration must be given to the proper selection of process parameters to ensure adhesion, uniform deposition, and proper composition (hardness and lubricity). The corrosion resistance of the actual refractory coating is typically quite good. I'll spare you the (phenomenally interesting) reasons why right now. The application and use of these coatings poses an interesting engineering problem - we are applying an extremely hard (and consequentially BRITTLE) material over a comparatively soft base material (our pistol). So, when the frame (or barrel, or whatever) flexes during recoil or firing each the hard material is forced to flex more than it wants. This can (but not necessarily) result in fractures at the surface. Like I said, no free lunch!!!
So, to summarize thus far:
Composite (lets say TiCN) > Metals (Cr) > Polymer for wear resistance
Composite > Metals > Polymer for corrosion resistance (this is grossly simplified)
Polymer > Metals > Composite for cost/economics
Metals > Composites > Polymers for potential issues in reduction to practice (think hydrogen embrittlment, surface fractures, and especially cost of fixing!!!)
On a personal note - of the coatings mentioned thus far (excluding bluing), my choice is the polymer. I think it’s a better route for firearms. The others may be more interesting technically, but not nearly as practical.
"Chemical Coatings" - Parkerizing, nickel phosphate, carborizing, nitrocarborizing, and others
Perhaps the most common, and certainly a good one, is parkerizing. Parkerizing is a "conversion coating" in that the surface atoms of the carbon steel is forcibly oxidized by exposure to phosphoric acid (among other additives like zinc, copper, and misc. salts). This process is actually more similar to bluing than some owners of fine English shotguns would care to think. Also closely related to this process is chromate conversion (done with chromic acid rather than phosphoric acid), leaving a dense, resilient layer of trivalent chromate (Cr+3). Very commonly used in aerospace, but quite toxic too. Also related to this is nickel triphosphate, which will be discussed on its own due to the unique status it holds in firearms coatings. Back to parkerizing . Parkerizing (and its related coatings) have some unique properties that lend themselves to anti-corrosion coatings. First, it provides an excellent adhesion layer to steel. Most epoxies do not adhere well to clean steel. Consequently, the better polymer coating people often use this as a pretreatment prior to applying their polymer system. The epoxy can actually crosslink directly to the coating, giving a wonderfully resilient coating. Also, parkerizing in particular, is somewhat "porous" in nature. This allows the absorption of oils and greases. If you pay attention to people who are into parkerizing (there are actually people who think this is attractive!), you will find people who when they get a newly parkerized pistol, will promptly douse the pistol in grease and throw it in the oven for a while to allow the grease to "melt in" to the parkerizing. Overall, this technology is still around because it works, and works well. It doesn’t stop corrosion, but does a good job of slowing it down so if you even attempt to care for your 1911, it will look good for a long time. Parkerizing doesn’t provide any meaningful wear resistance. It will wear from bearing point rather quickly.
Electroless nickel, aka nickel triphosphate, is a particularly interesting process. I am a fan of this done correctly. Electroless nickel is deposited by the catalytic reduction of nickel ions with sodium hypophosphite in acid baths at a set temperature (typically 80C or so). The deposits contains 3 to 13% phosphorus by weight. The phosphorus content significantly affects the coatings properties. As was mentioned before, unlike parkerizing, EN can be heat treated for increased wear resistance. Generally there are three types of EN (not including the potential additives like PTFE to give a single example). These are:
1. Phosphorus content between 3 and 7%. Hhigh wear resistance and corrosion resistance. Expensive to apply due to higher nickel content in bath.
2. Phosphorus content between 9 and 12%. Good compromise in cost and performance.
3. Phosphorus content between 10 and 13%. These are the "softest" and provide some of the best anti-corrosion properties.
Nickel-boron, which is closely related to nickel phosphate, is very often used in industrial wear applications for its high as-applied hardness. The boron content can be varied from 0.1 to 10%. Combinations of nickel, boron or phosphorus and other metals such as cobalt, iron, molydenum, aluminum, chromium, and other refractory metals are often used. These coatings are easily alloyed or blended with additional materials to improve friction, wear, and corrosion properties. PTFE is perhaps the most common addition. It has the effect of reducing the coefficient of friction of the coating. In newer systems, both "soft" and "hard" particles are often added. One particularly attractive system is a 10% phosphorus content with 10% PTFE and 5% SiC nanoparticles, resulting in a good compromise of wear, corrosion, and slickness. Also, and not commonly discussed, is the ability to change its appearance. Electroless nickel can be matte, semi matte, and shiny in appearance. It almost universally is grayish with a yellow hue. Some like the look. In addition to a silver color, EN can also be deposited in a light grey-charcoal-black form. This is accomplished by not adding the anti-oxidation chemicals in the bath. From a process perspective, EN is attractive because it doesn’t require a power supply. It also coats extremely uniformly, as opposed to hard chrome, which has a tendency to build up on corners and sharp projections.
Second, and finally today, carburizing and nitrocarburizing. There is a flurry of activity in this field. Lots of brand names, not many specifics. So I specifically won't be discussing anyone's specific process. Iron undergoes, similarly to many metals, unique changes when carbon (in the right form!) into the act. Now would be a good time to point out the nitrogen is close to carbon on the periodic table, and should be expected to impart similar properties to iron (and consequently steel), and it does. In fact, we have carburizing, nitriding, as well as nitrocarburizing. I will leave it to you to assign which process uses what. From my perspective, it isn't difficult to figure it out. The big names here end in -nite and -ifer. They are used on ugly plastic guns and high end custom 1911s. I like the look. Quite a bit actually. It can be applied to carbon and stainless steels with good effect. There are any number of ways to get the carbon and/or nitrogen into the steels. One can do it with a salt bath or with a plasma or with a vapor. As far as I know, these treatments got their start with gears. Feel free to correct me, but that is my impression. Obviously, "gear engineers" are after the same thing we are - a wear resistant surface that wont corrode and provides some degree of native lubrication. And they do. You will hear about nitriding in the automotive field. In fact, I have a Porsche crankshaft that I had nitrided. Basically, at somewhat high temperature, carbon and nitrogen can diffuse through the steel, forming iron carbide, iron nitride, and iron carbonitride. Because you have just oxidized the iron (from elemental Fe), there is a tendency not to further corrode. This is not to say it is impossible, but its darn difficult to do so. Performance wise this may be one of the better coating schemes out there, save for one caveat. These coatings will all compromise the fracture toughness of the part. You can’t stuff more atoms into a surface without creating some residual tensile stress. And, a good rule of thumb is the harder something is, the more brittle it is. Hardnesses run in the high 600's on the Vickers scale, with numbers dropping as you progress further through the coating into the underlying steel. These coatings typically drop CoF values 2-5 times over that of chrome plating.
Will continue later.
There are several reasons to coat metals (like a 1911) – aesthetics, corrosion protection, and wear protection. While the first category is obviously subjective, the second and third are much less so – and are often interwoven. Because aesthetics are subjective, I will only offer that charcoal bluing is the “be all end all” of bluing. Nothing else looks as good. Only rust bluing comes close. Hot blue is ugly. This is opinion, feel free to disagree all you like. Bluing by the way, provides nearly no protection from corrosion or wear. It is basically, in any of its iterations, simply a forced oxide layer growth. Few metals (Cr and Al come to mind) form a real “passivation” layer (corrosion engineers term to describe a naturally stable oxide formation preventing further oxide formation). The iron in steel isn’t one of them. Stainless steel gets its corrosion resistance from a high Cr content, typically 15ish% and up. It’s actually the Cr on the surface that passivates the steel, the stainless steel guys term that “throw.”
Corrosion and Wear Protection. We will only treat the corrosion and wear protection of steel (and in particular carbon steel) – these topics for aluminum and titanium are quite “deep” and fraught with nuances that are difficult to explain fully. That and 1911s are steel. Most of what is said can be transferred to stainless, and to a lesser extent, aluminum. Ti is a whole world in and of itself.
Ok – so we have two issues. One, we don’t want our $3k SRP to wear out. Two – we really don’t want it to ever rust. We have a several of potential routes available to us to remedy the situation. These include metallic coatings (eg – chrome and nickel); polymer coatings (eg – epoxy-PTFE); composite (eg – TiN, as in drill bits and that gawd aweful gold plating some people insist on, or electroless nickel); chemical, really two types – surface (eg – parkerizing, electroless nickel) and penetrating (eg – carborizing/carbonitriding); and other. The “other” is added to include those people who claim to be a ceramic coating or whatever.
The first category (metallic coatings) is perhaps the best and worst at the same time. Chrome is king here. There are other metallic coatings and claddings used, such as nickel. But none are as popular as good old industrial hard chrome. Chrome is almost universally deposited electrochemically in a bath of (among other things) hexavalent chromate. Consequently, that puts the plater up high on the EPA Hate List. There are other ways to do it, but are rarely ever seen (if at all) in firearms. This chemical (Cr2O7) is extremely toxic to people and the environment, and is the reason chrome coatings won’t be around forever. Chrome can be deposited thick enough to close up machine tolerances. When done right, will last forever. When done wrong, it results in cracked parts (like frames and slide stops), blistering, peeling, and sub chrome layer corrosion. This typically happens when the plater gets into a rush and deposits too quickly, resulting in the evolution of hydrogen gas at the surface of the part – leading to hydrogen embrittlement, holidays (defects), and uneven coating thickness. This is typically because of too thick of a coating or poor pre-cleaning. Some engineers (I’m one) argue that there is no way to prevent hydrogen embrittlement as a result of chrome plating. As a consequence, you will typically see a long term post bake (typically 400F or so) in a vacuum for critical parts. This forces the hyrdogen gas in the metal to evaporate from the metal (really cool phenomena actually). One can get two pieces of valuable information from this empirically – 1) chrome plating has its place (if the aerospace industry will tolerate the shortcomings, there is a reason!); and 2) chrome parts failure is common enough there is a procedure to minimize it. Nickel is close to chrome on the periodic table (same row, D block), and it stands to reason that the two metals behave roughly equally. They have roughly the same hardness and coefficient of friction. In fact, Cr, Co, and Ni were all there in the original “super alloys” for turbine engine blades. There are slightly different corrosion properties between the two, but suffice to say they are both better than plain carbon steel. As a gun guy (and an engineer), I really don’t like these coating systems. Too many ways to cause problems, and not that great rate of a return on wear and corrosion prevention. A point of clarification – here I am discussing elemental chrome and nickel coatings. On the plus side (as I see it), this is an ideal way to show off your gunsmithing talents – as chrome doesn’t hide ANYTHING!
The second category (polymer) has the greatest number of snake oil salesmen and BS floating around. One must keep in mind that polymer coatings are typically (and rightfully!!!) used in conjunction with surface chemical treatments. The most common surface treatments are: parkerizing (think Armor Tuff); silaninzing (don’t hear much about this in the firearms industry for some odd reason); electroless nickel; and chromate conversion coatings (again, not too popular, but for better reasons than silanes). These represent the worst in wear resistance, as would be expected. Polymers are just soft compared with metals. There are some who say corrosion protection remains intact after the coating wears through. Unless the coating is an active anti-corrosion coating (such as a chromate conversion coating – CCC), this is simply untrue. What typically happens is the places the coating wears through simply are high wear areas, and through natural abrasion, doesn’t allow the formation of a visible layer of rust. It’s still there, but you can’t see it with the naked eye. Polymer coatings are sometimes (by hacks) used like body filler on a car – to hid mistakes. There seems to be a wide variation in the properties of the polymer coatings out there. This is partially ture – some use different fillers and the properties vary accordingly. But the basic epoxy chemistry, for the most part, is consistent across the board. For example, if Brand X is often cited as a “softer” coating, it likely has more Teflon particles in it resulting in a thicker and softer appearing coating. With polymer coatings, “hardness” and “thickness” are intertwined (at least as far as most shooters are concerned). There is little variation in the actual epoxy resin and hardener in use by most shops. One of the more clever (and better, IMO) variations is a system that system has an underlying electroless nickel (nickel phosphate) deposited before coating. Functionally, polymer coatings offer a good compromise of protection and economics. They are cheap to apply and reapply as compared with metal and composite coatings generally. Cerakote is one of the more distinct polymer coatings out there. The word around the campfire is that this has some ceramic additives to enhance wear performance. I called NIC and they wouldn’t tell me. So I put a sample through our HPLC-MS, and mostly figured out what it really is. This represents a 2-5x increase in wear resistance over simple PTFE filled epoxy. This type (polymer) represents the lower end of both corrosion and wear protection when compared against all the other systems. However, it is also the least expensive and easiest to maintain. These also don’t typically suffer from the shortcomings seen with other coatings, like hydrogen embrittlment, cracking, and difficulty in future gunsmithing.
Category three (thin films, aka hard coatings, aka composite coatings) represents the richest field of tribology in my opinion. The only real contender would be chemical treatments (like carbonitriding) in terms of surface hardness. There is, in my opinion, room for interpretation in the distinction between say carborizing and carbide coating. This is because of the application techniques more than anything. I will try not to stray into chemical modifications (carborizing) tonight. Rather, we will discuss primarily coatings in their more rigorous definition. A couple of observations - these coatings are almost universally ceramics. Which are often used in grinding wheels. So unless you have two smooth surfaces, you have created two abrasive mating surfaces. I trust the implications of that are obvious.
There are many different types of coatings. They are typically iron, aluminum, chromium titanium, boron, and carbon based. For example: iron carbide, aluminum nitride, titanium carbon-nitride, boron carbide, and diamond like carbon. More generically, we are talking about carbides and nitrides (and mixtures of these). There are two general modes of coating applications: Physical Vapor Deposition and Chemical Vapor Deposition. PVD is typically used to produce DLC and thinner films of carbides and nitrides. CVD is typically used to produce thicker films, and lacks the fine control of morphology and composition that PVD typically provides. PVD coatings are typically less than 0.0005in, and CVD coatings are typically less than 0.001in. Both of these numbers are on the HIGH end of the range. They all have one unique thing in common - extremely high hardness. This gives them their most attractive feature - wear resistance. They in fact, were often developed for the tooling industry. And when I say tooling, I mean cutting tools. Like lathe inserts. These tools were developed, among other things, to allow single point cutting of "hard materials". Hardinge (IIRC) coined the phrase "Hard Turning" to describe this process. Prior to the development of these cutting tools, the shaping of nickel/chrome materials was accomplished by almost exclusively grinding operations. This is because the cobalt "tool" steel couldn’t effectively cut these materials. Ironically, this resulted in the development of powder metallurgy, and in particular, Metal Injection Molding, to allow the production of the cutting tools. So next time someone chimes in and preaches the virtues of tool steel and the evils of MIM, remember the cutting tool used to produce his tool steel are often produced via MIM! So these coatings provide (potentially!!!) one piece of the equation - wear resistance. In fact, micro hardness of many PVD/CVD coatings hovers in the 80Rc range, where tool steel is around 60ish Rc (high speed steel is a little higher, 62-64Rc). Great - we now have our wear resistant coating system! Now something that typically isn't discussed by people, which is a frequent problem in firearms, is adhesion. Remember I said PVD allows better composition control - well, CVD typically bonds better. This is because CVD is a "hot surface" process. The part to be coated is heated (to 400-1200C) and a vapor phase reactant is introduced into the chamber. The molecules of the gas injected react on the surface of the hot part, forming the film. A result of the elevated temperature is a "diffusion" bonding of the coating to the part. Remember, there are NO free lunches! So, hardness is universally high for these coatings. However, lubricity (a function of coefficient of friction) is all over the map. It really depends greatly on the individual process.
The other side of our equation is corrosion resistance. Here is the rub of these coatings - if a "holiday" exists, sever corrosion can result in the underlying metal. However, done right, it provides excellent oxidation resistance. Typically, though, these coatings are “line of sight” deposition techniques, so coating the inside of a firing pin channel is difficult.
Some other considerations and thoughts - these are the most expensive to apply. And you will be getting a coating developed for the tooling industry, not the firearms industry. Aesthetically, these range from the beautiful to straight up ugly! They can go from a look similar to stainless/bare steel (CrN for example) to a charcoal black (TiCN) to dark black (DLC) to a gawd aweful gold (TiN). Careful consideration must be given to the proper selection of process parameters to ensure adhesion, uniform deposition, and proper composition (hardness and lubricity). The corrosion resistance of the actual refractory coating is typically quite good. I'll spare you the (phenomenally interesting) reasons why right now. The application and use of these coatings poses an interesting engineering problem - we are applying an extremely hard (and consequentially BRITTLE) material over a comparatively soft base material (our pistol). So, when the frame (or barrel, or whatever) flexes during recoil or firing each the hard material is forced to flex more than it wants. This can (but not necessarily) result in fractures at the surface. Like I said, no free lunch!!!
So, to summarize thus far:
Composite (lets say TiCN) > Metals (Cr) > Polymer for wear resistance
Composite > Metals > Polymer for corrosion resistance (this is grossly simplified)
Polymer > Metals > Composite for cost/economics
Metals > Composites > Polymers for potential issues in reduction to practice (think hydrogen embrittlment, surface fractures, and especially cost of fixing!!!)
On a personal note - of the coatings mentioned thus far (excluding bluing), my choice is the polymer. I think it’s a better route for firearms. The others may be more interesting technically, but not nearly as practical.
"Chemical Coatings" - Parkerizing, nickel phosphate, carborizing, nitrocarborizing, and others
Perhaps the most common, and certainly a good one, is parkerizing. Parkerizing is a "conversion coating" in that the surface atoms of the carbon steel is forcibly oxidized by exposure to phosphoric acid (among other additives like zinc, copper, and misc. salts). This process is actually more similar to bluing than some owners of fine English shotguns would care to think. Also closely related to this process is chromate conversion (done with chromic acid rather than phosphoric acid), leaving a dense, resilient layer of trivalent chromate (Cr+3). Very commonly used in aerospace, but quite toxic too. Also related to this is nickel triphosphate, which will be discussed on its own due to the unique status it holds in firearms coatings. Back to parkerizing . Parkerizing (and its related coatings) have some unique properties that lend themselves to anti-corrosion coatings. First, it provides an excellent adhesion layer to steel. Most epoxies do not adhere well to clean steel. Consequently, the better polymer coating people often use this as a pretreatment prior to applying their polymer system. The epoxy can actually crosslink directly to the coating, giving a wonderfully resilient coating. Also, parkerizing in particular, is somewhat "porous" in nature. This allows the absorption of oils and greases. If you pay attention to people who are into parkerizing (there are actually people who think this is attractive!), you will find people who when they get a newly parkerized pistol, will promptly douse the pistol in grease and throw it in the oven for a while to allow the grease to "melt in" to the parkerizing. Overall, this technology is still around because it works, and works well. It doesn’t stop corrosion, but does a good job of slowing it down so if you even attempt to care for your 1911, it will look good for a long time. Parkerizing doesn’t provide any meaningful wear resistance. It will wear from bearing point rather quickly.
Electroless nickel, aka nickel triphosphate, is a particularly interesting process. I am a fan of this done correctly. Electroless nickel is deposited by the catalytic reduction of nickel ions with sodium hypophosphite in acid baths at a set temperature (typically 80C or so). The deposits contains 3 to 13% phosphorus by weight. The phosphorus content significantly affects the coatings properties. As was mentioned before, unlike parkerizing, EN can be heat treated for increased wear resistance. Generally there are three types of EN (not including the potential additives like PTFE to give a single example). These are:
1. Phosphorus content between 3 and 7%. Hhigh wear resistance and corrosion resistance. Expensive to apply due to higher nickel content in bath.
2. Phosphorus content between 9 and 12%. Good compromise in cost and performance.
3. Phosphorus content between 10 and 13%. These are the "softest" and provide some of the best anti-corrosion properties.
Nickel-boron, which is closely related to nickel phosphate, is very often used in industrial wear applications for its high as-applied hardness. The boron content can be varied from 0.1 to 10%. Combinations of nickel, boron or phosphorus and other metals such as cobalt, iron, molydenum, aluminum, chromium, and other refractory metals are often used. These coatings are easily alloyed or blended with additional materials to improve friction, wear, and corrosion properties. PTFE is perhaps the most common addition. It has the effect of reducing the coefficient of friction of the coating. In newer systems, both "soft" and "hard" particles are often added. One particularly attractive system is a 10% phosphorus content with 10% PTFE and 5% SiC nanoparticles, resulting in a good compromise of wear, corrosion, and slickness. Also, and not commonly discussed, is the ability to change its appearance. Electroless nickel can be matte, semi matte, and shiny in appearance. It almost universally is grayish with a yellow hue. Some like the look. In addition to a silver color, EN can also be deposited in a light grey-charcoal-black form. This is accomplished by not adding the anti-oxidation chemicals in the bath. From a process perspective, EN is attractive because it doesn’t require a power supply. It also coats extremely uniformly, as opposed to hard chrome, which has a tendency to build up on corners and sharp projections.
Second, and finally today, carburizing and nitrocarburizing. There is a flurry of activity in this field. Lots of brand names, not many specifics. So I specifically won't be discussing anyone's specific process. Iron undergoes, similarly to many metals, unique changes when carbon (in the right form!) into the act. Now would be a good time to point out the nitrogen is close to carbon on the periodic table, and should be expected to impart similar properties to iron (and consequently steel), and it does. In fact, we have carburizing, nitriding, as well as nitrocarburizing. I will leave it to you to assign which process uses what. From my perspective, it isn't difficult to figure it out. The big names here end in -nite and -ifer. They are used on ugly plastic guns and high end custom 1911s. I like the look. Quite a bit actually. It can be applied to carbon and stainless steels with good effect. There are any number of ways to get the carbon and/or nitrogen into the steels. One can do it with a salt bath or with a plasma or with a vapor. As far as I know, these treatments got their start with gears. Feel free to correct me, but that is my impression. Obviously, "gear engineers" are after the same thing we are - a wear resistant surface that wont corrode and provides some degree of native lubrication. And they do. You will hear about nitriding in the automotive field. In fact, I have a Porsche crankshaft that I had nitrided. Basically, at somewhat high temperature, carbon and nitrogen can diffuse through the steel, forming iron carbide, iron nitride, and iron carbonitride. Because you have just oxidized the iron (from elemental Fe), there is a tendency not to further corrode. This is not to say it is impossible, but its darn difficult to do so. Performance wise this may be one of the better coating schemes out there, save for one caveat. These coatings will all compromise the fracture toughness of the part. You can’t stuff more atoms into a surface without creating some residual tensile stress. And, a good rule of thumb is the harder something is, the more brittle it is. Hardnesses run in the high 600's on the Vickers scale, with numbers dropping as you progress further through the coating into the underlying steel. These coatings typically drop CoF values 2-5 times over that of chrome plating.
Will continue later.