FigurĀ» 2. Tribology Program structure

higher speeds, power is transmitted through gear systems with higher efficiency.

A major effort at NBS in 1987 was the assessment of the thermo-oxidative stability characteristics of synthetic oils. Thermal gravimetric analysis (TGA) and differential scanning catorimetry (DSC) methods were developed for this work. Basic property data were accumulated on poly-a-olefins (PAO), phosphate esters (PE), polyol esters (TMP), alkylated benzenes (AB), silicone fluids (SF), fluorinated compounds (PTFE) and other esters, mineral oils and a polyphenyl ester (PPE). The PFPE and PPE are viable base fluids for operations in the 300-400'C range from an oxidative and thermal standpoint. The remaining base fluids show operational capabilities in the 200-300*C range. However, the types of oxidation or decomposition products formed are as important as the possible operating range. The use of additives can result either in an improvement or detriment to deposit-forming tendencies. The DSC has been used to show these tendencies by comparing the relative amount of secondary products formed.1

The substrate material is expected to play as much of a role as the base fluid and additives. Evaluations conducted with steel, inert and Al203 substrates do show some differences in the interactions occurring. This may be due to surface-area effects or the types of primary reaction products formed, or it may involve the lubrication mechanism. This is being further pursued.

The evaluation of engine oils to determine deposit-forming tendencies has required full-scale engine testing in both the gasoline- and diesel-engine industries. A simple test utilizing the DSC has been developed that appears to predict the same deposit-forming tendencies as those found in engines. Preliminary comparisons were made on several fluids tested in diesel engines at Cummins Engine and in the DSC at NBS. The relative rankings of the oils were the same. The DSC method, if verified, could result in significant manpower and energy savings as a result of reduced requirements for full-scale engine evaluation of oils.

At Pennsylvania State University, vapor deposition on ceramic and metallic surfaces was produced by the thermal and oxidative reactions of alkyl and aryl phosphate esters, polyol esters, polyphenyl ethers, synthetic hydrocarbons and mineral-oil fractions over the temperature range of 500 to >800'C. On quartz, silicon carbide and silicon nitride surfaces, the deposit rates were linear with time. On metals, phosphate ester deposition was catalyzed by iron- and copper-containing alloys. Nickel and cobalt showed little or no catalytic effect.

The mechanisms of iron and copper reactions with vapor-delivered phosphate ester were studied using surface analytical tools including SEM, EDAX, TEM and X-ray diffraction. These data show that iron and copper are transported up through the coating to produce the catalytic effect. In the case of iron, large amounts of Fe2P and Fe3C were initially formed on the surface. As the films became thicker the composition changed to Fe2P and Fe3C5. In the case of copper, the near-surface films consist of Cu, Cu2P and graphite, indicating a different transport mechanism for the copper which continues to act as a catalyst in coatings greater than 2000 molecular layers thick.

Vapor-delivered lubricants of alkyl and aryl phosphates, polyphenyl ethers and polyol esters at 370"C provide good lubrication in a four-ball wear test with steel balls. In many cases, the wear values associated with vapor-delivered-lubricant deposits are as good as or better than those from the same lubricants in the liquid phase at 75"C.

Several wear tests with steel balls on silicon carbide discs have been run on TCP vapor in nitrogen. Preliminary analysis shows low wear with evidence of TCP-iron reaction products surrounding the wear scar on the silicon carbide disc. The activation of the friction polymer (Fe-TCP reaction products) appears to be enhanced by the high delivery temperature and the limited lubricant quantity in the conjunction. This unique and pioneering program in vapor-phase delivery of lubricant is oriented to the critical upper-cylinder-wall regions of advanced heat engines for which at present there is no available means of lubrication.

Surface Modification

The surface-modification task currently supports three research projects: 1) development of hard coatings for cutting-tool applications, 2) development of ionimplantation and ion-beam-enhanced deposition processes for producing self-lubricating surfaces and 3) development of highly oriented, abrasion-resistant coatings.

The hard-coatings project conducted jointly by ANL, Borg-Warner Corporation (BWC), George Washington University (GWU) and UCLA is aimed at developing PVD techniques to deposit hard, wear-resistant coatings. While the program focused on the development of hard coatings for cutting -tool applications, it was envisioned that the processes developed would be applicable in other areas such as piston rings for the low heat-rejection-engine program. The use of a cutting-tool geometry was extremely convenient in terms of process development, coating characterization and performance evaluation. BWC and UCLA were responsible for the development of two separate PVD processes: activated reactive evaporation (ARE) at UCLA and high rate reactive sputtering (HRRS) at BWC. GWU was responsible for the development of wear models to predict the performance of the different coatings (nitrides and carbides of Ti, Zr and Hf) based on abrasive wear and thermochemical dissolution of the coating during metal-cutting tests. ANL was responsible for characterization of the coatings to determine the coating composition and microstructure.

Metal-cutting tests were performed by BWC and ANL to evaluate the performance of the coatings and the validity of the models.

As a result of this program, two commercially viable processes were developed to deposit adherent, wear-resistant coatings on iron-based substrates at temperatures below their annealing temperature. Cutting-performance tests indicated the HRRS coatings outperformed uncoated tool inserts by a factor of 3 to 10, depending on the coating composition. The ARE coatings outperformed uncoated inserts by a factor of 3. Process development was also initiated by BWC to deposit a ternary compound of (Hf,Ti)N on cutting-tool stock to obtain higher coating hardnesses and hence greater wear-resistance by solid-solution hardening in a manner similar to that observed for (Hf,Ti)C coatings. Initial results on the ternary coating indicated that solution hardening was observed but was dependent on film stoichiometry.

Scanning electron microscopy (SEM), transmission electron microscopy (TEM), Auger electron spectroscopy (AES) and ion-backscattering analytical techniques were employed to characterize the composition and microstructure of the coatings. The SEM and TEM analyses indicated the coatings were columnar in nature with no detectable grain-boundary porosity present in the films. AES analysis of the coatings revealed the coatings were of high purity. Preliminary ion-backscattering (RBS) analysis performed with helium and protons indicated that the use of protons greatly enhanced the detection of low-Z elements (C and N) in the coatings. Both RBS and AES are being utilized on identical samples to determine if the two independent techniques give comparable results. The emphasis in the characterization effort is now focusing on examination of wear-tested inserts to identify the wear mechanisms.

The surface-modification process-development project at ANL is a new initiative focusing on the use of ion-implantation and ion-beam-enhanced-deposition processes for tribological applications. Based on the recommendations of an advisory panel held at ANL, the project will focus on developing and evaluating IBED processes for producing low-friction, self-lubricating surfaces. The program is both theoretical and experimental in nature with the theoretical studies focusing on modeling the deposition and subsequent friction and wear processes. An ion-beam-enhanced-deposition facility is currently being incorporated into the ANL tandem linear accelerator facility. Like the vapor-phase delivery system discussed above, this effort is also focused specifically on lubrication of the high temperature, upper-cylinder-wall regions of proposed advanced heat engines for which at present no means of lubrication exist.

Friction and Wear

Projects in the current friction-and-wear task area are addressing the questions: What is it about the composition, microstructure and processing of materials that determines their friction and wear behavior, and

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