Design and fabrication of sub-millimeter scale gas bearings with tungsten-containing diamond like carbon coatings
Micro gas bearings of sub millimeter size (diameter of 500µm and length of 300µm) with micron clearances were designed and fabricated through X-ray lithography and Ni electroplating. Details of the fabrication processes for the micro gas bearings are described. Procedures to make X-ray mask and technical issues are discussed. Static and dynamic hydrodynamic characteristics of the micro gas bearings were assessed using Molecular Gas Lubrication (MGL) theory. Fabricated micro gas bearings had lower stability than plain circular gas bearings due to lower load capacity and poor gas damping. Improved bearing designs having non-symmetrical step geometry with deep axial groove were suggested. These bearings were predicted to have much higher load capacities and dynamic stabilities than the fabricated micro gas bearings. Micro gas bearings were tested via an air jet-driven micro turbine made of SU-8, press-fitted onto the shaft. A self-aligning assembly technique for the micro gas bearings (using capillary action of photo resist) was developed and proved to be very effective. Successful operation of micro gas bearings at 60,000 rpm was demonstrated under limited operating conditions. Tribological characteristics of Ni micro bearings and amorphous tungsten hydrocarbon (W-C:H) coated micro bearings were investigated in dry friction mode, using a newly designed micro tribo tester. 900nm thick uniform conformal coatings were deposited onto the inner surface of micro bearings. Chemical and mechanical microstructures were studied via X-ray Photoelectron Spectroscopy (XPS), Raman micro spectroscopy. Wear rates, mechanical and material properties, and other tribological characteristics of W-C:H coated Ni micro bearings were investigated and compared to those of uncoated Ni micro bearings. Uncoated Ni micro bearings, as deposited and annealed at 800o C, experienced severe wear and appeared inadequate for tribological applications. Micro bearings with low tungsten-containing (5% wt) hydrocarbon had higher wear resistance than micro bearings with high tungsten-containing (11% wt) hydrocarbon. During the wear test of the W-C:H coated micro bearings, a transfer layer formed on the counter steel shaft even under very small contact pressure, leading to low steady state friction and high wear resistance.