Date of Award
Master of Applied Science (MASc)
The objective of this research was to develop a bench scale apparatus (tribometer) for mimicking the friction and wear conditions on the rake face of a metal cutting tool. The motivation for this study was to save test material costs, reduce machine downtime for testing, increase the number of test replicates and effectively add a reliable testing tool to characterize metal cutting operations and coating performance. The tribometer provides useful results approximately 8 times faster than an industrial machine once all materials have been obtained.
This study focuses primarily on isolating and studying the adhesive conditions on the seizure zone of the rake face. The setup simulates subsurface plastic flow in high temperature, near seizure conditions. This unit was constructed as a modified Brinell hardness test. A spherical tipped pin (3mm diameter) made of tooling material and treated with a coating is loaded under high stress and high temperature into a flat disk made of the workpiece material. The disk then rotates. The pin is aligned such that it is on the axis of rotation of the disk to produce near zero seizure like velocity. A high resolution reaction torque sensor measures the friction stress. The test unit is capable of simulating normal stresses of 0-4GPa and temperatures from room temperature up to 700-800°C depending on the specimens and heating patterns required.
This study focused on coated and uncoated cemented carbide pins with 6% cobalt. The workpiece materials used were Ti6Al4V and Inconel 718. 2 cases were tested for each workpiece. First the uncoated pin was tested, followed by an AlTiN Xceed® coating from Oerlikon Balzers®. Research conducted on an industrial machine has shown positive correlations with the tribometer measurements and agrees with the findings of our colleague Dr. L.S. Shuster in Russia who uses a similar test setup. The wear of the coated samples was lower than the uncoated samples.
The coefficient of friction (COF) was found to be the most suitable parameter for calculating tool performance. In general, the tests completed by the MMRI and Dr. Shuster (Fox-Rabinovich, Yamamoto and Aguirre, et al. 2010) showed that COF in similar rake face seizure conditions of a cutting tool is in the range of 0.2 - 0.35. The MMRI tribometer COF is calculated as follows:
Here MTOT is the reaction torque measured, FN is the normal load applied and RIND is the outer radius of the disk print whether a spherical imprint or a flat circular scratch. Temperature was found to be the most important variable affecting friction and it was also the most difficult variable to accurately measure. The interface temperature and the temperature gradient within the workpiece/disk material are the most significant thermal variables.
Biksa, Andrew, "Tribological Characterization of Surface Engineered Tooling for Metal Cutting Applications" (2010). Open Access Dissertations and Theses. Paper 4325.
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