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1 2007

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Właściwości mikromechaniczne i tribologiczne stopów tytanu po azotowaniu i tlenoazotowaniu jarzeniowym

Kot M., Moskalewicz T., Zimowski S., Czyrska-Ftlemonowicz A., Rakowski W.

Tribologia

2007

nr 2

261-271

W artykule przedstawiono wyniki badań mikrostruktury i właściwości mikromechanicznych oraz tribologicznych warstw wytworzonych metodą azotowania i tlenoazotowania jarzeniowego na stopach tytanu Ti-6Al-4V oraz Timetal 834. Szczegółowe badania mikrostruktury przeprowadzone za pomocą TEM umożliwiły określenie składu fazowego oraz grubości poszczególnych stref warstwy wierzchniej. Wyznaczono gradient twardości i odporność na zarysowanie warstw oraz ich odporność na zużycie. Twardość warstwy azotowanej na stopie Timetal 834 wynosi ponad 2100 HV, a warstw tlenoazotowanych ok. 900 HV. Azotowanie jarzeniowe zwiększa odporność stopu Timetal 834 na zużycie przez tarcie ponad 130 razy. Odporność na zużycie stopu Ti-6Al-4V po tlenoazotowaniu jest taka sama, jak po azotowaniu. Dla stopu Timetal 834 jest prawie dwukrotnie niższa dla tlenoazotowania niż azotowania.

Titanium alloys are nowadays frequently used because of their high strength, low density and maximum working temperature up to 600° C (Timetal 834) and find many applications especially in aerospace, marine industry and as biomaterials eg. for endoprotesis. But they have found little use in mechanical engineering. The main problem for titanium alloys is their poor tribological properties, low hardness, low resistance to wear and tendency to galling. Titanium alloys are subjected to various surface treatments giving hard external layers to improve mechanical properties, reduce coefficient of friction and wear. Titanium alloys: two-phase Ti-6A1-4V and pseudo-alfa Timetal 834 after nitriding and oxynitriding under glow discharge have been tested. The microstructural analyses were performed using light microscopy (LM) and analytical transmission electron microscopy (TEM). Nitrided layer on Ti-6A1-4V alloy has thickness 130-200 žm. For timetal 834 nitrided layer is thinner 70-180 žm. TEM investigations of cross-sectional thin foils revealed graded character with sublayers: outermost delta-TiN followed by delta'-Ti2N, epsilon-Ti2N and nitrogen-rich alfa(N) solid solution. Oxynitrided layers are thinner - 10 žm and 40 žm for Ti-6A1-4V and Timetal 834 respectively. These layers have also graded character with TiO, epsilon-Ti2N nitrogen-rich alfa(N) solid solution sublayers. Microhardness, Young modulus, resistance to scratch and tribological properties have been measured for all layers. A Micro-Combi-Tester of CSEM Instruments was used to measure hardness and Young Modulus and scratch resistance. Microindentations have been done on cross-sectional samples using Vickers indenter and 50 mN load. Rockwell C indenter, 200 mm tip radius has been used for scratch tests. Wear tests were performed on the ball-on-disc equipment under 2N load, track radius r = 5 mm, linear speed v = 0,03 m/s. AI2O3 balls - 1 mm diameter have been used. Wear rate was calculated from wear profiles after 1800 cycles. Hardness of nitrided layers is higher than oxynitrided for both alloys. Delta-TiN sublayer on Timetal 834 after nitriding has hardness 2100HV0,005. Outermost sublayers TiO produced by oxynitrification have hardness 900HV0,005. Graded character of all tested layers lead to gradient of hardness, which is gradually lower and lower for next sublayers. All tested layers show very good adhesion. For nitrided layers neither fracture nor delamination have been observed for load up to 20 and 25 N for Ti-6A1-4V and Timetal 834 respectively. Small area spallation, shell shaped cracks and delaminations occurred above these loads. Cohesive cracks have been observed above 20 N and 24 N for oxynitriding layers on Ti-6A1-4V and Timetal 834. Nitrided layer on Timetal 834 has highest wear resistance (Ws = 6,7 [L. mm3/(N*m)]*10-6). For Ti-6Al-4V wear is two times higher and is similar to wear of both oxynitriding layers. Nitrification and oxynitrification improve wear resistance. Wear is 60-130 times smaller than for untreated alloys. Results of all performed tests show than nitrification and oxynitrification under glow discharge is a prospective method for titanium alloys treatment to obtain light elements with high strength and wear resistance.