Metallurgical Aspects of Reliability for Small Nitrided Parts 670650
Many spacecraft experiments and systems employ drive systems which contain gear trains, slides, rachets, and similar components subject to sliding wear. In order to inhibit or mitigate the wear, some manufacturers employ surface hardening processes, including nitriding. Cases of three such applications are reviewed. One involved rachets made of Nitralloy 135M on a spectrometer grating drive. These rachets suffered excessive embrittlement of the very small (10 mil height) rachet teeth. Another case involved Type 416 stainless steel gears in a small motor gear train. These were nitrided by a proprietary process and suffered flaking of the nitriding and chipping of the gear teeth during testing. A third case involved a pinion gear on an infrared scanner which also was made of Nitralloy 135M and nitrided to increase its surface hardness. This gear was subject to flaking of the nitrided case and also breakage of the teeth.
In all of these cases, metallographic examinations and microhardness measurements were made. These disclosed that the case hardening operation resulted in an excessively deep hardened zone which rendered the hardened parts extremely brittle.
Discussion is made of the principles of case hardening, of the internal stresses that are introduced, and of the difficulties of applying the conventional process for large piece parts to the very small piece parts of many spacecraft systems. Alternate solutions to the wear problem of sliding mechanisms are presented.