A two-step CO<sub>2</sub> laser-sustained plasma nitriding process for deep-case hardening of commercially pure titanium
Open Access
- Author:
- Kamat, Amar
- Graduate Program:
- Engineering Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 15, 2016
- Committee Members:
- Judith Todd Copley, Dissertation Advisor/Co-Advisor
Judith Todd Copley, Committee Chair/Co-Chair
Stephen M Copley, Committee Member
Albert Eliot Segall, Committee Member
Fan-Bill B Cheung, Outside Member
Adrianus C Van Duin, Outside Member - Keywords:
- titanium
laser-sustained plasma
laser nitriding
case hardening
wear resistance
laser materials processing - Abstract:
- Titanium and its alloys possess several attractive properties that include a high strength-to-weight ratio, biocompatibility, and good corrosion resistance. However, due to their poor wear resistance, titanium components need to undergo surface hardening treatments before being used in applications involving high contact stresses. Laser nitriding is a thermochemical method of enhancing the surface hardness and wear resistance of titanium. This technique entails scanning the titanium substrate under a laser beam near its focal plane in the presence of nitrogen gas flow. At processing conditions characterized by low scan speeds, high laser powers, and small off-focal distances, a nitrogen plasma can be struck near the surface of the titanium substrate. When the substrate is removed, this plasma can be sustained indefinitely, away from any potentially interacting surfaces, by the laser beam power and a cascade ionization process. This dissertation explores the unique effects of nitriding titanium in the presence of such a laser-sustained plasma (LSP) in a processing chamber open to the atmosphere, with the ultimate objective of forming wide-area, deep, crack-free, wear-resistant nitrided cases on commercially pure titanium substrates. First, nitriding experiments were conducted at three processing conditions with and without a prestruck LSP. From optical and scanning electron microscopy, weight measurements, and temperature measurements, it was found that LSP nitriding increased nitrogen intake into the titanium melt pool, reduced surface oxidation, and broadened the energy distribution without causing energy attenuation when compared to conventional laser nitriding; these effects were most pronounced at the highest scan speed and lowest off-focal distance. Next, the effect of processing conditions on surface and cross-sectional microstructures during LSP nitriding was systematically evaluated by studying twenty experimental cases at varying off-focal distances, scan speeds, and gas flow compositions (nitrogen diluted by varying amounts of argon). X-ray diffraction conducted on the top surface of the nitrided trails confirmed the presence of TiN and TiN<sub>0.3</sub> phases. Although nitrogen dilution by argon was found to be necessary to prevent crack formation, microstructural characterization and weight measurements revealed that argon addition limited nitrogen intake into the melt pool by reducing Marangoni convection in the melt pool. Elimination of surface cracks thus came at the expense of reduced nitrogen content, shallower melt depths, and a reduced control over the microstructure of the resulting nitrided layer. To overcome these problems, a two-step process was proposed and developed. In the first step, a prestruck nitrogen LSP was used to deposit a single nitrided trail on the substrate; in the second step, a prestruck argon plasma was used to remelt the nitrided trail laid in step one. The nitriding and remelting steps were modeled using an analytical heat conduction solution valid for a moving heat source. Remelting the nitrided trail was found to refine the microstructure and reduce solute segregation, resulting in the formation of deep, hard, homogenous, and crack-free nitrided cases. Optimal processing conditions required to tailor the case microstructure from: (a) a two-phase mixture of TiN<sub>x</sub> dendrites embedded in a titanium matrix, to (b) a uniform solid solution of nitrogen in titanium, α-Ti(N), were identified. Microstructure-property relationships were proposed, and values for the nitrogen flux into the melt pool and the efficiency of nitrogen intake were estimated. The nitriding-remelting treatment resulted in the formation of nitrided cases having depths up to 800 μm, and average Vickers hardness values in the range of 475-730 HV<sub>0.3</sub>. Selected single trail processing conditions were used to deposit multiple overlapping trails to increase surface coverage. For the same processing conditions, an increase in average hardness was observed in the overlapping trail runs compared to the single trail runs. Crack-free nitrided cases of average case hardness values up to 643 HV<sub>0.3</sub> and case depths up to 600 μm were deposited. Reciprocating ball-on-flat wear tests were conducted to assess their wear resistance. The two-step treatment was found to enhance the wear resistance of the base material by up to 80%.