In some examples, an article includes a substrate and a coating on the substrate. The coating includes a stabilized microstructure including Magnéli oxide phase including an oxide of at least one of W, Mo, Nb, Ta, or Re. In some examples, a technique may include forming a coating including a refractory metal on a surface of a substrate. The technique also may include heat treating the coating at a temperature between about 500° C. and about 700° C. to form a coating including a stabilized microstructure including Magnéli oxide phase. The stabilized microstructure including Magnéli oxide phase may include an oxide of at least one of W, Mo, Nb, Ta, or Re. In some examples, the coating including the stabilized microstructure including Magnéli oxide phase exhibits a coefficient of friction that is at least 25% less than the coefficient of friction exhibited by the as-deposited coating under similar conditions.

TECHNICAL FIELD

The disclosure relates to self-lubricating coatings.

BACKGROUND

Lubricants are used to reduce friction and associated wear where two components contact each other. In some examples, liquid lubricants may be used. However, in some implementations, use temperatures may result in degradation of the liquid lubricant, so a dry film coating may be used as a lubricant.

In some examples, dry film coatings may include constituents that result in a chemical change in the coating as the temperature of the coating changes. This may result in the coating displaying lubricating properties over a range of temperatures. However, because the lubricating properties are based on the chemical changes in the coating, the lubricating properties may not be stable over multiple thermal cycles (e.g., from low temperature to a high temperature and back to a low temperature).

SUMMARY

The disclosure describes self-lubricating coatings whose lubricating properties are substantially stable under thermal cycling. For example, the coefficient of friction of the coating may be substantially stable over multiple thermal cycles. As used herein, substantially stable under thermal cycling means that the coefficient of friction does not appreciably change during thermal cycling over a temperature range from at least about 0° C. to about 700° C. In some examples, the self-lubricating coating may include a stabilized microstructure including Magnéli oxide phase. A Magnéli oxide phase has a metal-to-oxygen ratio of MenOn−1and/or MenOn+1, where Me is the metal and O is oxygen. In some examples, the metal in the Magnéli oxide phase includes an oxide of a refractory metal such as W, Mo, Nb, Ta, or Re. The coating additionally may include at least one of N, P, S, or O. In some examples, the self-lubricating coating may further include at least one transition metal, such as Ti, V, Cr, Zr, Ru, Rb, Hf, Y, Mn, Fe, Co, Ni, Ag, Au, or Pt.

The stabilized microstructure including Magnéli oxide phase may be formed by heat treating and/or thermomechanically treating a coating including the at least one refractory metal and any other coating constituents. In some examples, the coating is heat-treated, the heat treatment may be performed at a temperature below about 800° C., e.g., below about 700° C. For example, the heat treatment may be performed at a temperature between about 500° C. and about 550° C. Heat treating and/or thermomechanically treating the coating may form stabilized microstructure including Magnéli oxide phase in the coating, and may reduce the coefficient of friction of the coating compared to the as-deposited coating. In some examples, heat treating and/or thermomechanically treating the coating and forming the stabilized microstructure including Magnéli oxide phase may reduce the coefficient of friction by at least about 25% compared to the as-deposited coating.

By forming the stabilized microstructure including Magnéli oxide phase, the coefficient of friction may be substantially stable over multiple thermal cycles within a range. In some examples, the range may be about 0° C. to about 700° C. Because of the substantial stability of the coefficient of friction, the coating including the stabilized microstructure including Magnéli oxide phase may be used as a dry film lubricant coating in applications where the components undergo thermal cycling during use, such has high temperature mechanical systems.

In one example, the disclosure describes an article including a substrate and a coating on the substrate. The coating includes a stabilized microstructure including Magnéli oxide phase including an oxide of at least one of W, Mo, Nb, Ta, or Re.

In another example, the disclosure describes a system including a first article and a second article. In accordance with this example, the first article includes a substrate and a coating on the substrate. The coating includes a stabilized microstructure including Magnéli oxide phase and defines a first surface. In accordance with this example, the second article defines a second surface, and the first article and second article are positioned during use such that the first surface contacts the second surface.

In another example, the disclosure describes a method including forming a coating including a refractory metal on a surface of a substrate. The method also includes heat treating the coating at a temperature between about 500° C. and about 700° C. to form a coating including a stabilized microstructure including Magnéli oxide phase. The stabilized microstructure including Magnéli oxide phase may include an oxide of at least one of W, Mo, Nb, Ta, or Re. In some examples, the coating including the stabilized microstructure including Magnéli oxide phase exhibits a coefficient of friction that is at least 25% less than the coefficient of friction exhibited by the as-deposited coating under similar conditions.

DETAILED DESCRIPTION

The disclosure describes self-lubricating coatings whose lubricating properties are substantially stable under thermal cycling. For example, the coefficient of friction of the coating may be substantially stable over multiple thermal cycles. As used herein, substantially stable under thermal cycling means that the coefficient of friction does not appreciably change during thermal cycling over a temperature range from at least about 0° C. to about 700° C. In some examples, the self-lubricating coating may include a stabilized microstructure including Magnéli oxide phase. A Magnéli oxide phase has a metal-to-oxygen ratio of MenOn−1and/or MenOn+1, where Me is the metal and O is oxygen. In some examples, the metal in the Magnéli oxide phase includes a refractory metal such as W, Mo, Nb, Ta, or Re. The coating additionally may include at least one of N, P, S, or O. In some examples, the self-lubricating coating may further include at least one transition metal, such as Ti, V, Cr, Zr, Ru, Rb, Hf, Y, Mn, Fe, Co, Ni, Ag, Au, or Pt.

The stabilized microstructure including Magnéli oxide phase may be formed by heat treating a coating including the at least one refractory metal and any optional coating constituents. In some examples, the heat treatment may be performed at a temperature of below about 800° C., e.g. below about 700° C. For example, the heat treatment may be performed at a temperature between about 500° C. and about 550° C. Heat treating the coating may form stabilized microstructure including Magnéli oxide phase in the coating, and may reduce the coefficient of friction of the coating. In some examples, heat treating the coating and forming the stabilized microstructure including Magnéli oxide phase may reduce the coefficient of friction by at least about 25% compared to the as-deposited coating.

By forming the stabilized microstructure including Magnéli oxide phase, the coefficient of friction of the coating may be substantially stable over multiple thermal cycles within a range. In some examples, the range may be about 0° C. to about 700° C. Because of the substantial stability of the coefficient of friction, the coating including the stabilized microstructure including Magnéli oxide phase may be used as a dry film lubricant coating in applications where the components undergo thermal cycling during use, such has high temperature mechanical systems.

FIG. 1is a conceptual and schematic diagram illustrating an example article10including a substrate12and a coating14including a stabilized microstructure including Magnéli oxide phase on the substrate12. As described below with reference toFIG. 3, coating14may be formed by depositing a coating including at least one refractory metal (32) on surface18of substrate12, followed by exposing the coating to a heat treatment and/or thermomechanical treatment to form the stabilized microstructure including Magnéli oxide phase.

Substrate12may be a substrate of an article10that, during use, is positioned to contact or intermittently contact a surface of another article. For example,FIG. 2is a conceptual and schematic diagram illustrating an example system20including a first article10and a second article22. First article10may be substantially the same as article10shown inFIG. 1, while second article22includes a surface24that may contact surface16of the coating14including the stabilized microstructure including Magnéli oxide phase. In this way, surface16of coating14forms a contact surface or bearing surface for article10with second article22. In some examples, article10may be used in a high temperature mechanical system, and may be exposed to elevated temperatures during at least some portions of the use of article10. For example, article10may be used in a gas turbine engine. In some examples, article10may be exposed to temperatures as high as about 700° C. during use. Substrate12may include a metal, an alloy, a superalloy, or the like. For example, substrate12may include steel, nickel, a Ni-, Co-, or Ti-based superalloy, or the like.

Coating14is on surface18of substrate12. Coating14includes a stabilized microstructure including Magnéli phase oxide. A Magnéli oxide phase has a metal-to-oxygen ratio of MenOn−1and/or MenOn+1, where Me is the metal and O is oxygen. A Magnéli oxide phase forms has an atomic structure similar to graphite, where there are multiple thin sheets of Magnéli oxide with low bond forces between the sheets. Because of this, the sheets may move relative to each other relatively easily, which may provide a relatively low coefficient of friction.

In some examples, the metal in the Magnéli oxide phase includes a refractory metal such as W, Mo, Nb, Ta, or Re. Coating14may include at least one of W, Mo, Nb, Ta, or Re, and may include two or more of W, Mo, Nb, Ta, or Re. Coating14additionally may include at least one non-metal, such as N, P, S, or O. In some examples, coating14may further include at least one transition metal, such as Ti, V, Cr, Zr, Ru, Rb, Hf, Y, Mn, Fe, Co, Ni, Ag, Au, or Pt.

At least some of the refractory metal in coating14may be oxidized to form the stabilized microstructure including Magnéli oxide phase. In some examples, at least some of the at least one transition metal, if present in coating14, also may be oxidized to form the stabilized microstructure including Magnéli oxide phase. In some examples, coating14may include stabilized microstructure including Magnéli oxide phase dispersed within a matrix of other material, such as non-oxidized refractory metal, non-metal, and/or transition metal.

In some examples, after oxidation of coating14to form the stabilized microstructure including Magnéli oxide phase, coating14may comprise a chemical composition and phase composition that is substantially unchanging under changes in temperature. For example, after stabilization (oxidation), the amount of stabilized microstructure including Magnéli oxide phase and the composition of the Magnéli oxide phase and the other phases present in coating14may not appreciably change under changes in temperature.

In some examples, coating14may be relatively thin. For example, coating14may define a thickness, measured in the direction substantially normal to surface18, on the order of several micrometers. In some examples, coating14may define a thickness, measured in the direction substantially normal to surface18, of less than about 25.4 micrometers (less than about 0.001 inch).

FIG. 3is a flow diagram illustrating an example technique for forming a coating including a stabilized microstructure including Magnéli oxide phase on a substrate. The technique ofFIG. 3will be described with concurrent reference to article10ofFIG. 1for purposes of illustration only.

The technique ofFIG. 3includes depositing a coating including at least one refractory metal (32). In some examples, the as-deposited coating may include a metastable structure, such as an amorphous structure or a nanocrystalline structure. A metastable structure in the as-deposited coating may facilitate transformation of at least some of the as-deposited coating form Magnéli oxide phase by oxidation. For example, the metastable structure may allow formation of the stabilized microstructure including Magnéli oxide phase by heating to temperatures below about 800° C. (such as below about 700° C. or between about 500° C. and about 550° C.), rather than the temperature greater than 1000° C. used to convert coarse crystalline structures to Magnéli oxide phase.

As described above, in addition to the at least one refractory metal, the as-deposited coating additionally may include at least one non-metal, such as N, S, P, or O; at least one transition metal, such as Ti, V, Cr, Zr, Ru, Rb, Hf, Y, Mn, Fe, Co, Ni, Ag, Au, or Pt; or both. In some examples, the as-deposited coating may include tungsten (IV) sulfide (WS2).

The as-deposited coating may be deposited using, for example, physical vapor deposition (PVD), sputtering, evaporation, cathodic arc deposition, chemical vapor deposition, painting, slurry coating, or paste application of nano-powders.

After the coating has been deposited (32), the coating may be exposed to a heat treatment and/or thermomechanical treatment to form the stabilized microstructure including Magnéli oxide phase (34). The heat treatment and/or thermomechanical treatment may oxidize at least some of the at least one refractory oxide in the as-deposited coating to form Magnéli oxide phase. Additionally, if at least one transition metal is present in the as-deposited coating, the heat treatment may oxidize at least some of the at least one transition metal to form Magnéli oxide phase. Magnéli oxide phase has a metal-to-oxygen ratio of MenOn−1and/or MenOn+1, where Me is the metal and O is oxygen.

In examples in which the as-deposited coating is exposed to a heat treatment, the heat treatment may be performed at a temperature of less than about 800° C. As described above, the amorphous structure or a nanocrystalline structure may more readily convert into stabilized microstructure including Magnéli oxide phase than a bulk crystalline structure. Thus, the temperatures used to convert the at least one refractory metal into stabilized microstructure including Magnéli oxide phase may be lower than would be used if the at least one refractory metal were deposited with a bulk crystalline microstructure. In some examples, the heat treatment to form the stabilized microstructure including Magnéli oxide phase may be performed at a temperature of less than about 700° C., such as between about 500° C. and about 700° C., or between about 500° C. and about 550° C.

EXAMPLES

Comparative Example

A coating consisting essentially of WS2was formed on a substrate using physical vapor deposition (PVD). The coefficient of friction of the coating was evaluated using pin-on-disk testing against a silicon nitride pin.FIG. 4is a diagram illustrating the coefficient of friction as a function of time for the WS2coating. Curve42illustrates the coefficient of friction during a first test at room temperature (about 25° C.). As shown by curve42, the coefficient of friction was initially low, increased abruptly, than began to level off at about 0.7. Curve44illustrates the coefficient of friction for the sample during a second test at about 540° C. As shown by curve44, the coefficient of friction was substantially the same for the duration of the test at about 540° C., at a value of about 0.5. The coefficient of friction for this sample was generally lower during the test at 540° C. than during the first test at room temperature. Curve46illustrates the coefficient of friction during a third test at room temperature (about 25° C.). As shown by curve46, the coefficient of friction was initially about 0.5, increased, than began to level off at about 0.95. The coefficient of friction during the third test (second at room temperature) was greater than the coefficient of friction during the first test at room temperature, indicating that the coefficient of friction for the coating consisting essentially of WS2was not stable under thermal cycling.

Example

A coating consisting essentially of WS2was formed on a substrate using physical vapor deposition (PVD). The coefficient of friction of the coating was evaluated using pin-on-disk testing against a silicon nitride pin.FIG. 5is a diagram illustrating a coefficient of friction as a function of time for an example coating including a stabilized microstructure including Magnéli oxide phase when tested at room temperature, an elevated temperature, and again at room temperature. Curve52illustrates the coefficient of friction during a test at room temperature (about 25° C.). The friction test producing the curve52was performed after triblogical testing at 540° C. without a stabilization heat treatment. As shown by curve52, the coefficient of friction was initially about 0.6 then increased to about 0.95 during the testing.

Curve54illustrates the coefficient of friction during a test at room temperature (about 25° C.) after a stabilization heat treatment. As shown by curve54, the coefficient of friction was about 0.6 for the duration of the test at room temperature after the stabilization heat treatment, after an initial spike in the coefficient of friction. Additionally, the coefficient of friction after stabilization was about 30% lower than the coefficient of friction without stabilization and after heating to 540° C. (curve52). Curve56illustrates the coefficient of friction during a test at about 540° C., after the stabilization heat treatment. As shown by curve56, the coefficient of friction was substantially stable for the duration of the test, with a value of about 0.6. Additionally, the coefficient of friction at 540° C. was substantially the same as the coefficient of friction at room temperature, after the coating was stabilized. This indicates that the coefficient of friction was stable under thermal cycling.