Transcript שקופית 1 - מעבר לחומר

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.
Introduction
Ni and its Industrial coatings
TiN coatings
Nickel alloys main uses:
Ni Super-alloys for high temperature application like jet
engine.
harsh corrosive environments in chemical plants.
Coating methods
CVD
Reactive PVD
The following figure show XRD scans from the sample surface after different coating times.

*()
30
40
60
(+)
• 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.
Coating procedures
30
S.S.
container
T= 750-10000C
S.S
container
T=850-9500C
N – monatomic nitrogen
N Ni (Ti rich coating) N
N
Ti powder
(+Iodine)
Cr
Cr
powder
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.
*
40
50
60
70


40
50
 - Ni
* - Ni3Ti
- NiTi (cubic)
+ - NiTi (monoclinic)
 - NiTi2
60
70
Total layer
Total layer
-31
1200
-31.2
1000
-31.4
800
800
-31.6
-31.8
850
600
900
-32
-32.2
400
750
-32.4
‫ליניארי‬
(800)
-32.6
200
-32.8
-33
0
0
5
10
15
8.8
20
9
9.2
9.4
9.6
9.8
10
104/T(K)
time, h
Layer
Characterization methods
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
*
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).
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.
•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.
*
Titanization 2h
lnK
2. Nitiriding
X2
1. Titanization
70
30
 * +
Experimental procedures
Ti
powder
(+Iodine)
50
Titanization 8 min
*
Research goals
Ni
specimen
900°C

TiN coating properties
High hardness (20 GPa)
Stable and inert in many environment
High lubricity
Good thermal and electrical
conductivity
Industrial coating of Ni
Pack cementation (aluminizing, chromizing, siliconizing).
Thermal spray (plasma, HVOF). M-Cr-Al-X coating.
XRD scans
Substrate
This research
18kJ/mole
Previous diffusion
couples research
Total layer 138kJ/mole
NiTi
136.3kJ/mole
NiTi2
163.8kJ/mole
138-142k/mole (5052 at % Ni
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 crosssection series the growth of the TiN and the Ni3Ti layers on the expanse of the NiTi and NiTi2 layers.
Ni
NiTi
Ni with
Ti diffusion coating
eutectic
NiTi2
Ni3Ti
Working
range
NiTi
TiN
Ni
Ni3Ti
PIRAC nitriding,
900°C, 5h
NiTi2
Results
Titanization
TiN
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.
Ni3Ti
Ni
PIRAC nitriding
900 °C, 16h
surface microhardness
3500
3000
HV
2500
TiN hardness
2000
1500
1000
500
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).
8 min
Ni foil
Ni
NiTi Ni3Ti
NiTi2
Ti diffusion
coating
16hr, 900C
Nitriding
4hr, 950C
Nitriding
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.
NiTi Ni3Ti
15 min
0
2500
Ni
2000
NiTi2
NiTi
Fe
Ni3Ti
Ni
4 hr
NiTi
NiTi2
Ti(Fe) Sol.
1500
HV
2 hr
1000
500
Ni3Ti
0
Fe-substarte
Ticoating
Nitriding
Summary
NiTi
NiTi2
2
NiTi
NiTi
Ti3Ni
N
i
Ni3Ti
Ni
1. Two stage titanization-nitriding process (based on PIRAC method) has been developed for the coating of Ni.
2. 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.
3. 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.
4. During the nitriding steps only thin TiN coating (1-2 m ) are formed.
5. The applicability of the titanization-nitriding process to iron- and copper-base alloys has also been
demonstrated.
6. The coating was found to dramatically increase the surface hardness.