Development of Novel Vanadium Carbide Coatings Using a Large Area, Non-Line-of-Sight Plasma Process, 18-9189

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Principal Investigators
James H. Arps
Kevin C. Walter

Inclusive Dates: 04/01/00 - Current

Background - Plasma immersion ion processing (PIIP) is a relatively new vacuum technology for the application of hard, wear-resistant coatings. Like conventional physical vapor deposition (PVD) methods, PIIP is used to deposit various coatings. The non-line-of-sight PIIP approach allows simultaneous treatment of large components and complex shapes without requiring component manipulation. Building on established capabilities in ion beam surface modification techniques, SwRI has committed to the advancement of PIIP technology. The recent availability of inexpensive, high-power pulsed modulator equipment and the inherent scalability of PIIP lend an economy to this surface-modification process that is difficult to match using other methods. 

Approach - The goal of this research program is to help substantiate SwRI's position as the leading U.S. practitioner of PIIP for industry through the development of key next-generation PIIP coating compositions and approaches. A primary object has been the development of proprietary and patentable super-hard vanadium carbide (VC) and VC-based multilayer coatings using PIIP. These coatings, if successful, will have a hardness and wear resistance that greatly exceed that which can be achieved by PVD and most other conventional coating methods without subjecting the base material to high temperatures, as in chemical vapor deposition or thermal diffusion (>500°C). A second objective has been to combine VC deposition with other established PIIP processes for diamond-like carbon and chromium oxy-carbide coatings. The combination of deposition processes for different coatings using the same set of equipment will highlight both the versatility of the process overall and the ability to enhance and customize properties (adhesion, toughness) to suit a wider number of industrial needs. A third objective has been to develop the means to apply both new and established coating formulations to high-aspect, down-hole geometries. The current inability to apply PIIP-type surface modification technologies to such restrictive geometries has limited the market penetration and industrial acceptance of a new process. This effort will attempt to use the versatility of plasmas to deposit coatings onto areas that are essentially inaccessible by other vacuum-based technologies.

Accomplishments - A premise of this work is that a VC coating can be deposited from an organometallic precursor. This coating should be nonreactive with molten aluminum (Al), enabling the coating to have applications in the aluminum (Al) die casting industry. VC coatings made by a thermal diffusion process show such nonreactive properties, but these coatings are inherently hydrogen free. The SwRI coating process, on the other hand, naturally incorporates hydrogen (H) into the coating since H is a major component of most organometallic precursors. Thus, a simple question is whether the presence of H will sufficiently change the coating reactivity with molten Al. Instead of spending significant resources on organometallic precursors to test this premise, the research team decided to use existing expertise and deposit a VCxHy coating using ion beam-assisted deposition (IBAD) and test the reactivity with molten Al. If the coating does not react with molten Al,  plans to use organometallic precursors to deposit these coatings would be continued. A coated and uncoated sample was placed in a tube furnace along with an amount of pure Al. The substrates were heated (10°C/min) under flowing Ar (150 cc/min) to 800°C and held there for 1.1 hours. The samples were then cooled to room temperature and removed from the furnace. The photo below (Fig. 1) shows the results of this test. The Al reacted with the uncoated 440°C (left sample), adhered to the surface, and had to be forcibly pried off the sample. The coated sample (right) was protected from attack, and the Al gently slid off the sample when it was tilted. The discoloration in the coating, which was covered by the Al during the test, appears superficial and does not indicate a substantial change in coating thickness. Given these promising results, this task will proceed with greater confidence.

Fig. 1. Photograph of the 440°C samples and the associated Al pellets from the VCxHy reactivity tests. The uncoated sample (left) reacts with the Al, which had to be pried off the sample. The VCxHy coated sample (right) did not react with Al, which was effortlessly removed from the surface.

Potential and existing clients have most often inquired about treatment of inner diameters of tubes, a common restricted geometry. Since the beginning of this work, we have believed that plasma generation inside the tube would be the biggest problem. In initial experiments, two main types of behavior were observed. At low voltage, low pressure, and low pulse frequency, a glow discharge was generated in the tube that resembled that expected for a planar target. These discharges, referred to as Type I, resulted in a dim glow and relatively low currents (Fig. 2a). At higher voltage, higher pressure, and higher pulse frequency, a higher intensity discharge results that is bright and results in higher currents. We believe this discharge is a classic hollow cathode effect, and refer to it as a Type II discharge (Fig. 2b).

(a) (b)

Fig. 2. Photographs of Ar discharges inside a 7.25-inch diameter tube under (a) Type I discharge conditions and (b) Type II discharge conditions.

Parameters (bias, pulse width, pulse frequency, pressure) were then varied to map out the condition for which Type I and II discharges occurred. Representative parameters for each discharge type, and for each gas, were then used to sputter clean and deposit a diamond-like carbon (DLC) coating. In general, a hollow cathode discharge (Type II) occurs at higher pressures, higher pulse frequencies and lower voltages for smaller diameter tubes. Fragments from a silicon wafer were placed along the inner diameter of the tube as witness coupons and used to determine the thickness and hardness of the deposited DLC coating. Initial measurements suggested that while the coating thickness could change considerably (25-50 percent) along the length of the tube, the DLC hardness was consistent with previous work on planar samples and varied along the tube by a smaller amount (10-20 percent). Additional experiments are planned to further optimize coating properties.

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