Development of Self-Lubricating Coatings via Cold Spray Process: Feedstock Formulation and Deformation Modeling

Open Access
Author:
Aggarwal, Gaurav
Graduate Program:
Engineering Science and Mechanics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 06, 2007
Committee Members:
  • Ivica Smid, Committee Chair
  • Albert Eliot Segall, Committee Chair
  • Bernhard R Tittmann, Committee Member
  • Robert Carl Voigt, Committee Member
  • Timothy John Eden, Committee Member
Keywords:
  • Cold Spray
  • Coatings
  • Modeling
Abstract:
Because of their low density, high specific strength and high stiffness, titanium alloys are one of the prime candidates for structural application often requiring specific tribological properties. However, their relatively high friction coefficients and low wear resistance are limiting their application over a wider temperature range. Various coatings deposited with technologies like- high velocity oxy flame (HVOF), detonation gun (D-Gun), electron beam physical vapor deposition (EB-PVD), etc., can improve wear performance and decrease corrosion damage. These technologies require high processing temperatures precluding the integration of thermally vulnerable lubricants. This research looks at a relatively new coating process called Cold Spray for self-lubricating coatings on Ti-6Al-4V alloys. Cold Spray can produce coatings without significant heating of the sprayed powder or substrate. The particles are in solid state as they hit the substrate, and the formation of coatings occurs mainly due to the kinetic energy of the particles. Therefore, the impact velocity plays an important role. Below a critical value, the particles can cause densification and abrasion of the substrate. The focus of this study is to design composite coatings for the cold spray process and determination of the critical velocity through finite element modeling. Different powders and feedstock formulation techniques are discussed in order to find an optimum formulation for self-lubricating coatings. A composite powder (Ni coated hBN) was found to be the best candidate for the feedstock. The deformation of composite particles upon impact on the substrate was modeled and compared to the experiments. A number of approaches involving different modeling platforms, particle-substrate geometries, and material models have been tried. This work presents the results of ANSYS (version 10.0) analysis using an axisymmetric model of the particle impact. Stress and strain distributions in the particle-substrate interface have been observed for a wide range of impact velocities (200 to 1000 m/s). The results are evaluated to predict particle size, lubricant content, and finally the critical velocities for composite particles during the cold spray process. For the first time, the cold spray process is used to deposit Ni-MoS2 and Ni-hBN self-lubricating coatings. The modeling results are matched with the experimental results to provide guidelines for composite coatings via cold spray processing.