266 Prakt. Met. Sonderband 46 (2014) 
It has been recognized that NiTi alloys possesses good wear resistance, which is and 2. EXP 
desirable property in some biomedical and engineering applications (medical guide wires, 
orthodontic wires, bone joints) [2-4]. It is well known that B2 austenitic parent phase exhibits 
an excellent wear resistance [5]. On the other hand, the wear resistance of the B19’ The exp: 
martensite phase is weak and must be improved for some applications. To improve the wear pure ace 
resistance many surface modification processes such as carburizing or nitriding could be of the lo 
used [6]. But the main restriction is in the high affinity of titanium to oxygen. To solve this publicatic 
problem, the surface modification process must be performed at high vacuum conditions. in the retor 
this research work we use low-pressure vacuum carburizing process for improving the wear annealin, 
resistance of NiTi shape memory alloys. predeten 
the retort 
Nowadays vacuum carburizing is a modern, very efficient and environmentally friendly for the pr 
process for case hardening of steels used in aerospace, automotive and industrial sector. the sam. 
The process of vacuum carburizing is performed under oxygen free hydrocarbon gas For the v 
atmosphere (usually pure acetylene) at partial pressure lower than 20 mbar and in and dian 
conditions which are far from thermodynamic equilibrium. It is a non-equilibrium boost- polishing 
diffusion-type carburizing process in which the steel being processed is austenitized in a to minimi 
rough vacuum, carburized in a partial pressure of hydrocarbon gas, diffused in low vacuum 
conditions, and then quenched either with oil or gas [7]. The vacuum carburizing is The thick 
performed in vacuum furnaces under a low pressure hydrocarbon gas atmosphere — usually a 
pure acetylene or mixtures with other gases [7-9]. The process begins with heating the parts analysis 
under vacuum to the predetermined carburizing temperature. Upon reaching a constant part consistec 
temperature, acetylene is introduced into the vacuum chamber up to adequate pressure, with dian 
and the samples are kept for the predetermined carburizing time. On completion of the of NiTi 
carbon adsorption process, the gas is evacuated and the furnace is put under vacuum again. microsco 
In the next step the adsorbed carbon diffuses from the surface into the matrix of the sample 
for a predetermined period of time to achieve the desired depth of the carburized zone. 
Finally, in the last step the annealing temperature is reduced to the quenching temperature. 
Gas quenching is performed in mainly nitrogen, helium, hydrogen or in a mixture of these 
gases. The desired case depth and corresponding concentration profile of carbon are 3. RESI 
obtained by diffusion of carbon atoms from the furnace atmosphere into the matrix by 
cyclical changing the saturated (active) and the diffusion (passive) stage of the process [7- 
9]. The vacu 
of the fi 
Vacuum carburizing has also found its way into other challenging applications such as in temperatı 
medical devices, where carburizing is performed on implant screws. The addition of carbon surface c 
produces a high strength, high hardness surface with excellent wear and abrasion dissolutio 
resistance [10]. inward th 
exceedec 
The main topic of our research work was to determine the optimal operating conditions of titan carb 
vacuum carburizing process for achievement of compact and homogeneous TiC film on the sample a 
surface of the NiTi alloy. Additionally, the influences of formed TiC on the microstructural TiC film is 
changes in the NiTi alloy were investigated. (fig. 1a ar 
In this res 
optimal o 
obtained 
gas (fig. 
laver (fig.