CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of U.S. Provisional Patent Application 61/622,692 filed April 11, 2012, the contents of which are included herein by reference. FIELD OF THE INVENTION

[0002] This invention relates generally to firearms. More specifically, the invention relates to firearms having a wear and friction reducing coating disposed on the inner surface of their barrels. Most specifically, the invention relates to firearms having a ceramic coating disposed upon the inner surface of their barrels by a chemical vapor deposition (CVD) process. BACKGROUND OF THE INVENTION

[0003] In use, the barrels of firearms are exposed to high pressure, high temperature, and high friction conditions which can cause significant wear, which in turn affects the accuracy and service life of the firearms. Military firearms typically employ high temperature, fast burning propellants which increase the muzzle velocity of projectiles but further exacerbate problems of wear. Significant barrel wear leading to failure can occur after no more than 10,000 rounds; and in some particular firearms, significant barrel wear occurs after firing no more than 1,000 rounds.

[0004] The prior art has implemented a number of approaches toward extending the service life of firearm barrels. In some instances, the inner surfaces of the barrels are treated by a nitriding process carried out in either a molten salt bath or by a plasma process; however, such processes are very difficult to implement, particularly in small caliber barrels since the aspect ratio of a typical barrel can exceed 70. Also, the typical rifling geometry of the inner surface further complicates an uniform access for most conventionally used coating processes. Hence, it is very difficult to form an adherent, uniform conformal coating under such conditions. In other instances, chromium coatings have been electroplated onto the inner surface of firearm barrels. While such coatings can work well to extend the service life of the barrels, the high aspect ratio of the barrels makes them very difficult to electroplate. Furthermore, chromium plating processes generate large amounts of waste effluent containing hexavalent chromium, which is an acknowledged health hazard and requires specialized handling. [0005] As a consequence, there is a need for a process for providing uniform, wear resistant coatings on the interior surfaces of firearm barrels, which - is relatively simple to implement and does not generate hexavalent chromium waste materials. As will be explained in detail hereinbelow, the present invention is directed to a chemical vapor deposition process, carried out at either low pressure or atmospheric pressure, which allows for the uniform coating of the interior surface of firearm barrels with wear resistant, friction reducing ceramic materials.

BRIEF DESCRIPTION OF THE INVENTION

[0006] The present invention comprises a barrel for a firearm, wherein the barrel has a coating of a ceramic material deposited on at least a portion of its inner surface. In particular instances, the coating is comprised of one or more of a nitride, carbide, boride, oxide, oxycarbide, oxyboride, boronitride, or oxynitride of a metallic material. In specific instances, the ceramic material may be one or more of chromium carbide, chromium nitride, titanium carbide, titanium nitride, titanium boride, and/or aluminum oxide.

[0007] A typical coating has a thickness in the range of 0.2-20 microns, and the coating may be deposited by a chemical vapor deposition process implemented at pressures ranging from subatmospheric to atmospheric. In some specific instances, the firearm barrel is fabricated from steel, the coating is comprised of chromium carbide, and at least a portion of the carbon component of the chromium carbide coating is derived from the steel. In a process of this type, the chromium carbide layer may be deposited from a process gas which includes at least hydrogen and a halogen such as chlorine. A typical coating process is carried out at a temperature in excess of 100°C, such as a temperature in the range of 500-1100°C.

[0008] In certain instances, the coating method may be integrated with an annealing and /or hardening step in a single process so as to eliminate the need for a separate production step of annealing and hardening. Following coating and hardening, the gun barrel may be subjected to a tempering step.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is an Energy-Dispersive X-ray (EDX) spectrum for an uncoated firearm barrel; and

[0010] Figure 2 is an Energy-Dispersive X-ray (EDX) spectrum for a firearm barrel coated with a ceramic material in accord with the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention is directed to firearm barrels which have at least a portion of their interior surface coated with a ceramic material by a chemical vapor deposition technique. The chemical vapor deposition technique may be carried out at atmospheric pressure, or at a pressure below atmospheric, in which instance it is termed a low pressure chemical vapor deposition (LPCVD) process. The ceramic material may in some particular instances be one or more of a nitride, carbide, boride, oxide, oxycarbide, oxyboride, boronitride, or oxynitride of a metallic material. In specific instances, the ceramic material may be one or more of chromium carbide, chromium nitride, titanium carbide, titanium nitride, titanium boride, and aluminum oxide. The thickness of the coating will depend on particular applications but in some instances ranges from 0.2-20 microns. In particular instances the coatings may comprise a number of layers, which may be of differing compositions.

[0012] In the chemical vapor deposition techniques of the present invention, a process gas which includes one or more precursors of the components of the ceramic material is introduced into the barrel of the firearm. The barrel and process gas are maintained at an elevated temperature causing the process gas to react and deposit the ceramic material onto the surface of the firearm barrel. The CVD process of the present invention is relatively simple to implement and control and is readily adaptable for the deposition of high quality, uniform, conformal ceramic coatings onto the inner surface of firearm barrels. As such, the process of the present invention is readily adaptable for the coating of small caliber, high aspect ratio barrels.

[0013] In some instances, all components of the ceramic coating will be derived from the process gas itself; while in other instances, some components of the coating may be derived from the material comprising the firearm barrel. For example, in particular implementations of the present invention, steel firearm barrels are coated with a chromium carbide coating by a chemical vapor deposition process in which the process gas includes a chromium compound (typically a chromium halide such as chromium fluoride) and the carbon component is derived from carbon present in the steel. Processes of this type may likewise be used to form other metal carbides and oxycarbides.

[0014] Process conditions will depend upon particulars of the ceramic material being deposited. In some instances deposition is carried out by a LPCVD process typically carried out at pressures below atmospheric and in specific instances at pressures below 200 millibar. In some specific instances, deposition is carried out at a pressure in the range of 80-90 millibar. In most instances, the coating reaction is carried out at temperatures above ambient, such as temperatures in excess of 100°C. In particular instances, the coating process is carried out at temperatures in the range of 500-1100°C; and in some specific instances, the coating is carried out at a temperature of approximately 800-900°C.

[0015] The process of the present invention may be implemented utilizing various combinations of process gases and under a range of deposition conditions as discussed above. In one illustrative example, a group of firearm barrels was coated with a coating of chromium carbide. In this particular process, the chromium component of the coating is derived from the process gas itself while the carbon component is derived from the steel.

Experimental:

[0016] The purpose of this experimental series was to deposit and evaluate a ceramic protective coating on the inside diameter of a 0.223 caliber rifle barrel. The rifle barrels used in this series have an aspect ratio (length/diameter) exceeding 70. The barrels include a rifled bore surface and the object of the experimental series is to demonstrate that the coating could be applied so as to cover and follow the rifling of the surface. Coating was carried out by a low pressure chemical vapor deposition process (LPCVD). In this process, parts to be coated are heated to a processing temperature and one or more volatile precursors are introduced at specified flow rates and pressure into the reactor containing the parts. The precursors then react homogeneously or heterogeneously and undergo one or more of: pyrolysis, hydrolysis, reduction, carbide/nitride formation, or the like to form a solid coating layer on the part. It was found that the use of low pressure conditions avoids the occurrence of unwanted homogeneous or gas-phase reactions and promotes coating uniformity. In this experimental series, chromium carbide (CrC) was used as the coating material, although it is to be understood that other ceramic compositions may likewise be utilized.

[0017] The LPCVD system used in this experimental series was a Bernex 425L apparatus fixtured to support a number of rifle barrels therein. Other apparatus known in the art may likewise be employed.

[0018] The precursor materials for the coating process included ferrochromium (FeCr) in the form of pellets and this material functioned as a source of chromium. The gaseous precursors used in the process comprise hydrogen, which functions as a carrier and a reducing gas; argon, which functions as a carrier/diluent and aids in maintaining thickness uniformity; methane, which prevents decarburization of the steel substrate; and hydrogen chloride, which aids in maintaining chlorination of the FeCr.

[0019] The substrates to be coated comprised 10 rifle barrel blanks of 4000 series steel alloy including rifled bores. In order to simulate a full coating run in the apparatus a number of steel blanks were also utilized. These blanks were formed of mild steel and had dimensions similar to that of the rifle barrels but did not include any rifling formed therein.

[0020] A first coating cycle was carried out with 4 actual barrels and 48 dummy barrels in the apparatus. The barrels were first cleaned to remove exterior surface oxidation and loaded into the reactor. The actual barrels were placed so as to be aligned along a radial axis of the cylindrical reactor to determine the uniformity of the gas flow in the process. Approximately 2 pounds of FeCr were loaded into the reactor which was then sealed. The parts were preheated to a temperature of approximately 650°C under a reducing atmosphere provided by a hydrogen gas flow of 3.0 L/min while the reactor pressure was maintained at 200 mbar. Once temperature was reached, the parts were held for 60 minutes so as to allow for uniform heating, and hydrogen flow rate and pressure were maintained during this time. Thereafter, the parts were further heated to a coating temperature of approximately 880°C and methane gas was introduced into the reactor at a flow rate of 1.0 L/min to prevent decarburization of the steel substrate. The hydrogen flow rate and pressure were maintained as previously. These conditions were maintained for 20 minutes to further allow for uniform heating of the parts, and argon was introduced into the reactor at a flow rate of 5.0 L/min while hydrogen and methane rates were maintained as before. The pressure in the reactor was transitioned during this time from 200 mbar to the coating deposition pressure of 100 mbar.

[0021] Hydrogen and argon flow rates were maintained from the previous step, and methane was replaced by hydrogen chloride at a flow rate of 1.0 L/min. In the process, the hydrogen chloride gas reacts with the FeCr and forms chromium chloride. During this step, the chemical interactions at the parts' surface include the reduction of chromium chloride by hydrogen, leading to synthesis and deposition of chromium on the parts' surface and diffusion of elemental carbon from the substrate into the chromium layer to form chromium carbide. This portion of the step lasted for approximately 180 minutes.

[0022] Thereafter, the hydrogen chloride and argon gas flows were discontinued and the reactor was purged by hydrogen gas at a flow rate of approximately 3.0 L/min with pressure being maintained at approximately 100 mbar. Thereafter, hydrogen flow was maintained and the pressure raised to atmospheric so as to allow the parts to cool to room temperature. Toward the end of the cooling step, as a safety precaution, the hydrogen gas was replaced with nitrogen gas. Once the parts were cooled to room temperature, the reactor was opened and the parts removed for analysis.

[0023] Analysis of the parts included identification of the composition of the CrC coating, and measurement of its thickness profile and distribution throughout the barrel, and measurement of the hardness of the barrel substrate.

[0024] In order to determine the coating thickness and distribution the barrels were cut along their length. The first 6 inches of a barrel is considered the most critical section as it is subjected to the maximum wear and erosion during the firing process. Samples were polished and then etched using a 4% Nital etchant to differentiate between substrate and coating. The coating thickness, morphology, and composition were determined using a scanning electron microscope (Tescan Series 2). Hardness was determined using a Rockwell hardness tester (Instron). Environmental effects of the coating process were determined by collecting process effluents resulting from cleaning of the reactor base and downstream components of the system following the coating process.

[0025] Table 1 below shows the average coating thickness measurements for one of the barrels at the inlet side at 2 inches, 4 inches, and 6 inches.

Table 1

The coating thickness distribution inside the barrel indicates that the coating gets thinner with increase in distance from the inlet side, and it will be seen that the average coating thicknesses at 2 inches from their inlet side is around 2.0 microns which indicates uniform distribution of process gas in the radial direction of the reactor. Electron micrography indicates excellent coating adhesion to the barrel accompanied by conformal step coverage of the rifled surface. The composition of the coating was determined by Energy-Dispersive X-ray (EDX) spectroscopy for both the barrel substrate and the coating layer. This analysis was carried out at 15 KV accelerating voltage. Figure 1 shows the EDX spectrum for the uncoated substrate and Figure 2 shows the corresponding spectrum for the coated substrate. Presence of the chromium and carbon peaks in the EDX spectrum for the coating confirms that the coating composition is CrC. The aluminum and copper peaks in this spectrum are artifacts of the aluminum sample holder and copper mounting tape used in the scanning electron microscope chamber.

[0026] The average as-coated substrate hardness of the barrels was measured at approximately 26 HRc. The processing conditions used for the coating of these barrels are similar to the annealing conditions used for the barrel material and the measured hardness is in line with the hardness expected after annealing of the barrels. Analysis of the effluents of the coating process confirms that they do not contain any environmentally hazardous chemicals at levels over regulated permissible exposure limits therefore confirming that the coating process is an environmentally safe option to prior art processes such as electroplating.

[0027] Following this first experimental series, a second series of barrels were coated and incorporated into functioning firearms for field testing. The process for coating the second series of barrels is essentially similar to that described above except that the argon flow rate was changed from 5.0 L/min to 5.5 L/min so as to improve the thickness distribution of the coating in each barrel and the total coating time was increased by 17% so as to increase the overall coating thickness.

[0028] Following coating, the barrels may optionally be subjected to further processing, including polishing, heat treatment and the like as desired. It has been found that heat treating can provide for a more uniform microstructure and higher hardness in the coated barrel as compared to like coated barrels which were not subject to a heat treatment process. As mentioned above, the heat treating step may be carried out following coating; and in one instance, such a heat treating process involves subcritical annealing, hardening (with a pressure quench), and tempering in a vacuum furnace. Tempering may be done at 650°C at a holding time of approximately 2 hours. Analysis of samples thus treated shows a more uniform microstructure and a final substrate hardness between 29.5 HRc and 34 HRc as compared to a hardness of approximately 26 HRc for the as-coated, non heated treated substrates.

[0029] In other instances, the chemical vapor deposition technique of the present invention may be integrated into the basic barrel fabrication process. For example, barrels are often annealed and hardened (austenized) at elevated temperatures, these steps may be carried out in conjunction with the chemical vapor deposition coating step. Consolidation of these steps will save time and costs, since the coated barrels will then only have to be tempered after the CVD treatment instead of annealing, hardening and tempering. [0030] The coatings of the present invention have been found to significantly enhance the service life of firearms by decreasing barrel wear. In addition, the coatings of the present invention have been found to increase muzzle velocity of projectiles fired from the firearm, and this is believed to be the result of reduced friction and reduced material transfer from the bullet to the inner surface. The coatings of the present invention may be applied to the firearm barrels when they are originally manufactured, or the coatings may be applied to barrels which have previously been used so as to thereby restore them back to acceptable operating standards.

[0031] While the foregoing invention was described with regard to some specific embodiments, it is to be understood that other embodiments, modifications, and variations will be readily apparent to those of skill in the art in view of the teaching presented herein. The foregoing drawings, discussion, and description are illustrative of the principles of the present invention but are not meant to be limitations upon the practice thereof. It is the following claims, including all equivalents, which define the scope of the invention.