F. A. Nichols

Solid Lubrication of Ceramic Surfaces by IAD-Silver Coatings for Heat Engine Applications

In this study, we investigated the tribological characteristics of alumina ceramics that were coated with silver by means of ion-assisted deposition (IAD). Tests were performed at temperatures up to 400°C on an oscillating-slider wear test machine as a partial simulation of a ring/cylinder system. The results showed that higher test temperatures caused greater wear damage and increased friction of uncoated alumina. At 400°C, the wear rates of the alumina-alumina test pairs were eight to ten times higher than those of specimens tested at room temperature. The steady-state friction coefficients of the uncoated alumina were also rather high, typically ranging from 0.8 to 1.1. In contrast, the wear rate of I AD-silver -coated flats was negligible and the wear rate of counterface pins sliding against these coated flats was reduced by factors ranging from 4 to 73 depending on test temperature. Friction was also reduced to about one-half to one-third that of uncoated pairs. Surface and structure analytical inves...

Tribological behavior of oil-lubricated, TiN-coated steel

Abstract The effects of titanium nitride (TiN) coatings on the tribological behavior of M50 and 52100 steels under both dry and oil-lubricated conditions were evaluated with a reciprocating sliding-pin-on-flat test machine. Under dry conditions, the TiN coating reduced wear, which occurred by abrasion and by oxidation of the sliding surface. It also reduced the amount of wear-debris accumulation at the contact interface. During oil lubrication, wear and roughening of the contact area (usually associated with the boundary lubrication regime) was eliminated by the TiN coating. The formation of a boundary film by chemical interaction between the oil additives and the wearing surface was also prevented by the TiN coating.

Effect of Metallic-Coating Properties on the Tribology of Coated and Oil-Lubricated Ceramics

Friction and wear behavior was determined for zirconia ceramics lubricated with solid coatings (Ag, Au, and Nb) deposited by ion-beam-assisted-deposition (IBAD) techniques, and a polyol-ester-based synthetic oil. Although the use of soft Ag and Au coatings as solid lubricants in conjunction with the synthetic oil significantly reduced the fiction and wear under boundary lubrication at temperatures up to 250°C, these films had poor durability. In contrast, the Nb coating was more durable in terms of chemical reactivity and adhesion during the tribo-tests than were the Ag or Au films. However, the friction and wear behavior of the Nb-coated zirconia was poorer than that of the ceramics coated with Ag or Au.

The role of soft (metallic) films in the tribological behavior of ceramic materials

The friction coefficient and wear rate in structural ceramic materials are often too high to make them attractive for applications requiring dry sliding. The role of a thin soft silver film in improving the tribological characteristics of silicon nitride materials is investigated in this study. In a pin-on-disc contact configuration in reciprocating motion, the friction coefficient was reduced by about 50% and the wear rate by one to two orders of magnitude. The friction reduction is thought to be due to the reduction of the contact interface shear strength by the presence of the soft film. The wear rate reduction is likely due to modification of the contact stresses by the film. Combined, these two factors lead to a decrease in the magnitude of the damaging tensile stresses. The durability and thus the effectiveness of the film is very dependent on the bond strength (i.e., adhesion strength) between the film and the substrate. 21 refs., 4 figs., 1 tab.

Sliding Friction and Wear of Ceramics With and Without Soft Metallic Films

In unlubricated sliding contact, essentially all the mechanical work done to overcome friction is converted into heat produced in the vicinity of real contacts. The amount of frictional heat flux q is proportional to the friction coefficient u, the normal force F , and the sliding velocity ν, but is inversely proportional to the nominal contact area A n (e.g., q = (u × F × ν)/ A n ). The real areas of contact, being much smaller than the nominal contact area, give rise to much higher local heat fluxes in the vicinity of asperity contacts. Because the frictional heat flux enters the contacting bodies through these regions (or locations known as “hot spots”), their local temperatures (referred to as “flash temperature”) can be much higher than the overall or “bulk” surface temperature, as discussed in References 1-3. Previous studies have demonstrated that frictional heat can profoundly affect the friction and wear behavior of both metallic and ceramic materials. In most steels and nonoxide ceramics, frictional heat was found to foster oxidation. The occurrence of phase transformations on or near the sliding surfaces was also cited in the literature for certain steels and ZrO 2 -based ceramics. Except for SiC, BeO, and AlN, most ceramics have significantly lower thermal conductivity than do metals. When in sliding contact, ceramics cannot dissipate frictional heat generated at sliding interfaces as effectively as most metallic alloys. Large temperature gradients can often develop between areas of real contact and surrounding regions, thus creating high thermal stresses.

Weibull Analysis Applied to the Pull Adhesion Test and Fracture of a Metal-Ceramic Interface

Various tests have been developed to measure the mechanical bonding or adhesion of thin coatings deposited on substrates. In the pull test, pins that have been bonded to the coating under test are pulled with increasing force normal to the coating until the coating is pulled from the substrate. For many systems, large scatter in the data is often observed due to uncontrolled defects in the interface and the brittle nature of the pull test. In this study, the applicability of Weibull statistical analysis to the adhesion of Ag coatings to vacuum sputter-cleaned zirconia was examined. Data were obtained for smooth and rough substrates for various levels of adhesion. A good fit of the data to the Weibull distribution was observed. The Weibull modulus was found to depend on the roughness of the substrate but was insensitive to the adhesion strength. This seems to be the first reported application of Weibull statistical analysis to pull test data. Presented at the 48th Annual Meeting in Calgary, Alberta, Canada...

Boundary film for structural ceramic materials

Abstract Structural ceramic materials, like metals, will require lubrication if they are to be used extensively for tribological applications. The use of thin soft metallic coatings (specifically Ag) as a boundary film during mineral oil lubrication of silicon nitride (Si 3 N 4 ) and zirconia (ZrO 2 ) ceramic materials was investigated in this study. With a pin-on-flat contact configuration in reciprocating sliding, the steady friction coefficient was reduced by a factor of two (0.14–0.16 vs . 0.06–0.07) when the flats were coated with Ag. Also, with Ag coatings, the wear of pins was reduced to an unmeasurable level, whereas, in the absence of Ag coatings specific wear rates of about 2×10 −9 -4×10 −8 mm 3 N −1 m −1 and about 7×10 −8 -2×10 −7 mm 3 N −1 m −1 were measured for Si 3 N 4 and ZrO 2 pins respectively. In addition to preventing direct contact between pins and flats, thereby reducing wear, the Ag coatings also act as a solid lubricant, help dissipate flash heating, and accelerate modification of the λ ratio.

Comparison of Various Sink Strengths for Analyzing Radiation Creep, Growth, and Swelling

The essential physics involved in the reaction-rate-theory analysis of radiation effects at temperatures where both vacancies and self interstitials are mobile is contained in the expressions used for the strengths of distributed point-defect sinks such as dislocations, cavities and grain boundaries. These sink strengths have been obtained by various authors in distinctly different ways, thus giving rise to some possible confusion in comparing the various results. This is even more true with respect to the effect of interaction fields on these sink strengths and the so-called bias factors or sink efficiencies have been defined in entirely different ways, thus rendering quantitative comparisons difficult. We present here a comparison of several procedures in the literature, and attempt to make reasonable quantitative comparisons.

Asymmetric dislocation/point-defect interactions and the modeling of void swelling

An internally consistent model has been developed for analyzing void swelling during irradiation. The model employs Wigner-Seitz cells around each type of sink, namely dislocation lines, voids, and grain boundaries. Uniform generation of vacancies and interstitials is accounted for, as is diffusion in response to both concentration and radial interaction field gradients. The cells are coupled by equating concentrations and current flows at their peripheries, for arbitrary densities of the various types of sinks. A procedure has been developed for obtaining the best radial interaction fields to replace the actual angularly dependent ones surrounding dislocations. Analytical theoretical support for the chosen value (-1/4 of the average of the square of the actual field) has been developed. Quantitative accuracy has been assessed by comparison with numerical studies employing full angular dependence. The predicted swelling-rate bias factor of approx. 50% is in excellent agreement with available swelling-rate data when one assumes that approx. 16% of the defects initially generated escape close-pair recombination within the cascade. Reasonable theoretical support exists for this survival fraction.