Engineered Self-Lubricating Coatings Utilizing Cold Spray Technology

Open Access
Author:
Stark, Lisa M
Graduate Program:
Engineering Mechanics
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
November 16, 2010
Committee Members:
  • Dr Al Segal, Thesis Advisor
  • Albert Eliot Segall, Thesis Advisor
  • Ivica Smid, Thesis Advisor
Keywords:
  • Electroless Nickel
  • Cold Spray
  • wear
  • friction
  • boron nitride
Abstract:
It is often beneficial to modify the surface of engineered components to gain desirable properties, such as improved wear resistance. One fabrication method is to apply a coating with these desirable properties. Self-lubricating coatings can improve the performance of contacting surfaces and extends a part’s lifetime by reducing the coefficient of friction and specific wear rate. In this study, self-lubricating coatings of hexagonal boron nitride particles in a nickel matrix were investigated and optimized to improve wear resistance. These self-lubricating coatings were fabricated by high velocity particle consolidation (Cold Spray) of hexagonal boron nitride powder encapsulated by nickel and nickel-phosphorous alloys. The nickel encapsulation, via electroless nickel plating, gives the particles desirable properties for the cold spray process. In fact, this ductile nickel encapsulation protects the brittle boron nitride and allows particle deformation, which is critical for the success of cold spray deposition. The resulting nickel coating contains imbedded lubricant particles, which are released during intimate contact with a surface. The release of the lubricant particles reduces the interaction between mating materials and the potential for wear. Hexagonal boron nitride particles have been successfully encapsulated through the electroless nickel deposition procedure resulting in powder of 20 micron mean particle size. The powder was successfully deposited by cold spray on aluminum 6061 substrates. The wear properties of the coatings were tested and compared to a pure nickel cold sprayed coating. The results showed that incorporating an optimal amount of hexagonal boron nitride in the nickel coating will reduce the coefficient of friction and the wear rate. Coatings were tested via reciprocating ball-on-flat wear, sliding pin-on-disk wear, and fretting. Hexagonal boron nitride particles embedded in the coating were characterized through optical microscopy, scanning electron microscopy, energy dispersive x-ray spectroscopy, focused ion beam, micro hardness, and density.