A multivariate tribological evaluation of candidate materials, surface treatments and dry film lubricants is necessary for design of moving mechanical components that function reliably in extreme conditions, including for long duration space missions. In this study, linear reciprocating or unidirectional sliding friction data was collected using ball-on-flat tests. The balls were hardened 440C stainless steel (either uncoated or sputtered with MoS₂) and flat surfaces were 440C stainless steel, Nitronic 60 stainless steel or Ti6Al4V titanium alloy with various surface treatments and/or dry film lubricants. Surface treatments included anodizing, nitriding and electrical discharge machining. The dry film lubricants included Microseal 200-1 , sputtered MoS₂  and a nano-composite coating i-Kote. The data contains applied normal load, measured friction force, calculated coefficient of friction, ball position, ambient temperature and relative humidity during testing. Tests were performed at different peak Hertzian contact pressure conditions ranging from 300 MPa to 2000 MPa. Data is also available for flat surfaces that were vacuum baked at 150 ^oC after surface treatment and dry film coating as well as samples tested in inert gas (nitrogen) environment. This data can be used both to fundamentally understand the tribological properties of different material systems as well as to enable design of components for specific applications, conditions and duty cycles.

Test Conditions

Tribological testing to measure friction and wear was performed using an Rtec Instruments Multifunction tribometer, modified to include a custom enclosure that enabled testing in inert gas environments. Linear reciprocating and unidirectional sliding tests were performed to measure coefficient of friction (CoF) with ball-on-flat and ball-on-disk set-ups using modified ASTM G-133 and ASTM G99-17 standard parameters.

Most tests were performed at room temperature (22 °C) and ambient air conditions with relative humidity between 30-50% and some tests were performed in dry nitrogen environment at room temperature and relative humidity < 5% and oxygen concentration < 0.4%. The test parameters in the reciprocating tests were 10 mm stroke length, 0.5 mms^-1 sliding speed and applied loads corresponding to calculated peak Hertzian contact pressures of 300, 500, 800, 1100 and 2000 MPa. In the unidirectional tests, the sliding speed was 1000 mms^-1 at 300 MPa and 100 mms^-1 at 1100 MPa contact pressure.

Materials

Flat or disk specimens with different substrate materials and surface treatments were tested. The substrate was stainless steel (440C or Nitronic 60) or titanium alloy Ti6Al4V. The Ti6Al4V alloy consisted of machine cut substrate (referred to as ‘Ti6Al4V’) and electrical discharge machining (EDM) cut substrate (referred to as ‘EDM Ti6Al4V’). Surface roughness measurements of the substrates before treatment or DFL coating were made using contact-mode profilometry and roughness of the tested surfaces (included in the Overview spreadsheet) was measured using interferometry. The surface treatments included in this study were nitriding and anodizing, and the dry film lubricants tested were sputtered MoS₂, Microseal 200-1 or i-Kote. The anodized surfaces included in this study were fabricated by two different vendors Danco (Type 2 anodizing) or Tiodize (Type II and Type IV anodizing).

440C stainless steel balls of diameter 4.76 mm (3/16 inch) or 9.53 mm (3/8 inch) and Rockwell C60 hardness (with and without sputtered MoS₂) were used as the counter-body.

Friction Data Processing

The available files contain raw (reciprocating and unidirectional tests) and post-processed (reciprocating tests) data. Post-processing was done in the case of reciprocating tests to correct for friction force bias that occurs due to misalignment between the force transducer and the sample surface. This misalignment is typically a result of changes in surface topography during sliding or machining imperfections. Misalignment induced friction force measurement uncertainties can be minimized by averaging the absolute friction force from the forward and backward motion of the ball on the sample. Averaging and data smoothing was accomplished using a moving mean and Bartlett window size of ~ 8000 data points with a boundary handler. Although, the Bartlett window function can result in discontinuities at the beginning and end of the data, the general CoF trend is not affected.

In most cases, there are only two test repetitions and in, few cases, three.

Not all pressure conditions were tested for all material/surface treatment/dry film lubricant cases. 

In a few tests, temperature data was not collected, and the temperature is shown as `-999’.

Wear data was not captured in this study.