TiN coating on Ni alloys by reactive surface modification

1. Ni3Ti NiTi Ni Fe Ni3Ti NiTi NiTi2 Ti(Fe) Sol. Ni NiTi NiTi2 Ni3Ti Ni NiTi NiTi2 Ni3Ti  XRD scans 900°C Substrate  *() Titanization 8 min * * * * (+) Titanization 2h •  - Ni • * - Ni3Ti • - NiTi (cubic) + - NiTi (monoclinic) •  - NiTi2 NiTi2 NiTi Ni3Ti Ni     * + S.S. container S.S container T= 750-10000C T=850-9500C Ti powder (+Iodine) Ti powder (+Iodine) N – monatomic nitrogen Ni specimen NN N Ni (Ti rich coating) Cr Cr powder eutectic Working range Ni NiTi Ni with Ti diffusion coating NiTi2 Ni3Ti NiTi TiN Ni PIRAC nitriding, 900°C, 5h Ni3Ti NiTi2 TiN PIRAC nitriding 900 °C, 16h Ni3Ti Ni 8 min 15 min 2 hr 4 hr NiTi2 NiTi Ni3Ti Ni TiN coating on Ni alloys by reactive surface modification Mashall I., Gutmanas E.Y., Klinger L. & Gotman I. Technion – Israel Institute of Technology, Faculty of Material Engineering, Haifa. Abstract In the present research, a two-stage titanization-nitriding process has been developed with the goal to grow a wear- and oxidation-resistant TiN coating on the surface of nickel alloys. The process is based on the Powder Immersion Reaction Assisted Coating (PIRAC) method originally developed for the coating on non-oxide ceramics. At the first stage, Ni surface was enriched in Ti by PIRAC annealing in a halogen-activated Ti powder. This resulted in the formation of a multi-layer coating consisting of different Ti-Ni compound layers. The treatment temperature did not exceed 900ºC to avoid liquid formation (the low melting Ti-Ti2Ni eutectic). The kinetics of different layers growth at 700-900ºC was found to be controlled by diffusion and obeyed the parabolic growth law. The activation energies of the process as a whole and of separate layers growth have been measured. The effect of different processing parameters (amount of halogen, shape and size of Ti powder, etc.) on the formation of Ni-Ti surface layers has been studied. The Ti-enriched surface was subsequently transformed into a titanium nitride layer by PIRAC nitriding – a process based on annealing the sample under a low nitrogen pressure formed by selective diffusion of the atmospheric nitrogen. The thickness of the TiN layer did not exceed 1 m even after 16 h long PIRAC treatment at 900ºC. During nitriding, the significant growth of the Ni3Ti layer and the shrinkage of the Ti2Ni and NiTi layers (all formed during the titanization procedure) were observed. Several peculiarities of layers growth were observed both during titanization and during nitriding PIRAC procedures. The applicability of the TiN PIRAC coating process to Iron- and copper-base alloys has also been demonstrated. The following figure show XRD scans from the sample surface after different coating times. • Research goals • To develop wear- and oxidation-resistant TiN coating on the surface of nickel alloys. • To study the kinetics and mechanisms of reactive diffusion involved in the formation of Ti diffusion coating and its nitriding by the PIRAC method. • To further the knowledge on reactive diffusion process in the Ni-Ti system. Experimental procedures Coating procedures 1. Titanization 2. Nitiriding The kinetic parameters of the Ti diffusion coating by the activated PIRAC process were studied (Ni substrate in Ti sponge with 2 wt. % I): experiments were performed at 4 temperatures and 4 exposures at each temperature and thickness of the layers as function of temperature and exposure time was measured. The results correlate with a parabolic growth law and the parabolic relation was used to estimate the activation energy for the growth of the total layer and of the specific layers. Part of the data is presented at the next figure. As can be seen for the 900ºC the parabolic law is not obeyed because the non-parabolic growth of the Ni3Ti layer (as was found in earlier works). A simplified model was proposed to explain the phenomena (not shown here). A schematic of activated PIRAC (Titanization) process is presented at the above Figure. Ni specimens are immersed into Ti powder with Iodine (2% in most cases) and placed into sealed Cr-rich stainless-steel container the chrome react with the oxygen and maintain oxygen vacuum in the pack. This container is placed into second S.S. container with Ti and Iodine: Ti is acting as a getter, reacting with nitrogen to avoid nitriding of the Ti powder in the first container with the specimen. Iodine prevents leakage of Iodine from the first container. The third S.S. container with a small addition of Cr powder acts as an additional getter reacting with oxygen and preventing oxidation of the Ti powder during the coating process. In the second stage (Nitriding) The Ni samples after Ti diffusion coating, are placed into sealed stainless steel (S.S.) container that is placed into another sealed S.S. container containing Cr powder acting as a oxygen getter. Because the small affinity of Cr to N the nitrogen diffuse through the S.S. container to react with the Ti enriched layer on the sample surface. • Characterization methods • XRD scans of specimen surface • SEM including SE and BSE micrograph of surface and coating. • EPMA for chemical analysis of surface and at cross-section including line-scans. • Optical microscope and chemical etching of cross-section and surface. • Microhardness at surface using loads of 50-200gr with Vickers intender. The following table show the results obtained. The activation energy is for the temperature range of 750-850ºC Ni-Ti phase diagram – showing the temperature working range and the expected intermatallic Nitriding After the charactization of the titanization process the nitriding step was established The coating surface and a cross-section of the growing layer can be seen in following figures. In the cross-section series the growth of the TiN and the Ni3Ti layers on the expanse of the NiTi and NiTi2 layers. Results Titanization First experiments were made without activator. No coating was found. Because of this an activator was added to the pack (Iodine) as can be seen at the coating procedure above. The resulted coating surface is presented at the following figure. Microhardness of the coating is presented in this graph, showing the dramatically improved coating hardness. TiN hardness SEM (BSE) micrographs of the growing coating at 900ºC is presented at the following figures with a general line scan sample of the cross section (800ºC, 4 hr). After the successful coating formation on Ni substrate, the same method was used to coat other substrate as copper and steel. The titanization of the steel (1020) and the microhardness measurements form the surface is shown. Summary Two stage titanization-nitriding process (based on PIRAC method) has been developed for the coating of Ni. The process includes the reactive diffusion of Ti into a Ni substrate resulting in the formation of intermetallic layers as a first stage, and nitriding of the Ti-rich intermetallic layers with the formation of TiN coatings, as a second step. The kinetics of layer growth during titanization was found to be parabolic in the range of 750-850ºC with activation energy of 138kJ/mole similar to other works on NiTi diffusion couples. During the nitriding steps only thin TiN coating (1-2 m ) are formed. The applicability of the titanization-nitriding process to iron- and copper-base alloys has also been demonstrated. The coating was found to dramatically increase the surface hardness.