FIELD OF INVENTION
This invention relates to a process for coating a substrate using
a non-plasma sprayable grade ceramic composition.
This invention further relates to a process for coating a
substrate using a non-plasma sprayable grade ceramic composition
for metal substrates, using commercially available and
inexpensive ceramic powder.
BACKGROUND OF INVENTION
Surface engineering is one of the important areas in interdis-
ciplinary engineering sciences. The object is to endow the
surface of a component with a property that the base material
does not have. An interesting example is the case of a lathe bed.
The base material is cast iron; it is cheap and has reasonable
stiffness and high damping capacity. However, it exhibits stick
s1ip properties when the 1 athe c a rr i age mo ves on i t, c aus i ng
inaccuracies in '.slide movements. To circumvent this problem, the
bed is clad by a thin layer of bronze reinforced PTFE. This
cladding results in a bed with good stiffness and adequate
damping properties in the bulk and anti stick-slip property on
the surface. Thus, the properties of the base materials are
complimented by those of the clay layer.
The surface modification process involves chemical/thermochemical
treatment on the surface to bring about microstructural changes,
followed by application of a coating/clay layer having appro™
p r- i a t e p rop e r t :i e s»
The former has its limitations. On the other hand, the latter is
considered to be a very versatile tool for surface modification.
The type of coating to he provided depends on application. There
are many techniques available, eg. electroplating, vapour
depositions, thermal spraying, etc.. Among these techniques,
thermal spraying is popular for its wide range of applicability,
.•adhesion of coating with the substrate and durability. Most of
the thermal spraying processes are so called cold processes which
minimises substrate damages. Plasma spraying is one such
versatile and technologically sophisticated thermal spraying
technique. The process can be applied to coat on variety of
substrates of complicated shape and with a range of sizes using
metallic, ceramic or polymeric consumables. The production rate
of the process is very high and the coating adhesion is also
adequate. Since the process is almost material independent, it
has a very wide range of applicability, eg. thermal barrier
coating, wear resistant coating, etc.. Thermal barrier coatings
B.re provided to protect the base material, eg. IC engines, gas
turbines, at high temperature. Z:i. re on i a (ZrO ) is a conventional
2
thermal barrier costing material. As the name suggests, wear
resistant coatings &r-e used to combat wear, especially in
cylinder liners, pistons, valves, spindles, textile mill rollers,
etc.. Alumina { calcia powder, mixing and ball
milling are done simultaneously.
Plasma spraying of alumina—2ireon has been carried out on low
carbon to high carbon steel, cast iron and some alloy steel
substrates successfully. It has been observed that the coatings
without bond coat peeled off immediately highlighting the fact
that the substrate gets oxidized rapidly. The use of Ni-Al bond
coat offers 3 better adhesion to the substrate as it reduces the
thermal mismatch between the mild steel and the ceramic coating,
thus minimising the oxidation of the substrate. The objective of
the present study, to obtain a good wear resistant and thermal
barrier coating has been achieved.
The coatings have been characterised by far XRD, wear, thermal
fatigue, grindability and SEI"J. The XRD results show the formation
of multiple phase which exhibits excellent thermal shock
resistance along with a good wear resistance.
RESULTS
Cross sectional view of PDZ, zircon and zircon—20 wt54 calcia
coatings are shown in figures l(a—c). In all micrographs between
substrate, bond coat, (Ni-Al) and the top coat are cle&rly
visible. It can be noted that no crack or discontinuity is
present at an of these interfaces.
It is quite clear from this cross section metallography study that interfaces between the
substrate, bond coat, and top coat are quite continuous and sound No spalling or
separation is observed. One of the major difficulties encountered during processing and
also during elevated temperature exposure is the diffusion of elements between top
coat, bond coat, and substrate. Although a limited amount (few atomic distances) of
diffusion is required to achieve a high interfacial bond strength, a long range diffusion
renders the coating ineffective. It is interesting to note that in alumina (monolithic) and
Zr02 + Si02 (two phase) coatings, a long range diffusion is not taking place This is
evident in the x-ray dot map obtained using EDS technique in a SEM A typical example
is shown in figure % where the distribution of various elements in the cross section of a
PDZ/NiAI/MS specimen is presented It is quite clear that no long range diffusion of Zr,
Si, Ni and Fe has taken place across the interfaces.
Similar cross sectional views have also been studied in the case of coatings with high
carbon iron (HCI) bond coat. Typical SEM micrographs of the PDZ and zircon top coats
having HCI bond coat are shown in figures3(a) and$(b), respectively It may be noted
that boundaries between the bond coat and the substrate m.-jy not be highly
distinguishable owing to the fact that diffusion of iron and carbon may take place across
the bond coat/substrate interface. However, the interfaces between top coat, bond coat
(HCI), and substrate appear to be quite sound as no crack is visible
Typical polished top coats of PDZ, zircon and zircon 20 wt% calcia are
shown in figures 4(c-e). Owing to a very high temperature of melting,
ZrO„ and SiO? remain as two separate constituents of the ceramic coating.
From the XRD study, it is ob served that all the crystalline 5phases poly—
morphs) of zirconia are existing, but with varied amounts. Owing to a
rapid solidification associated with the plasma spraying process, silica
develops an amorphous structure in general. Then one can visualise this
coating as a ceramic-ceramic composite coating containing zirconia and
silica. The two-phase mixture can be clearly observed in the microstruc-
ture. It is a matter of concern that a coating with a two-phase micro-
structure may not be as sound as a monolithic one. However, zirconia-
silica two-phase coating offers an acceptable wear resistance and attrac-
tive thermal fatigue properties.
Table 2 shows the response of various types of ceramic coatings
to cyclic: thermal loading which is also known as thermal fatigue
behaviour. The behaviour- of thermal barrier coatings under
thermal shock depends strongly on heating/cooling conditions and
micrestructure of the coating. Heating/cooling rate and maximum
and miniumum temperature are the two most important factors
controlling the number of cycles to failure and the failure
mechanism. A monolithic coating may have better resistance to
thermal, shock load than a two-phase coating.
Differences in the thermal expansion coefficients between top
coat, bond coat and substrate alongwith diffusivities of various
elements present in them are also responsible for controlling the
thermal fatigue behaviour of such materials. From table 2, it
appears that specimens with Ni--5% Al bond coat survived thermal
cycling better than those with HC1 bond coat. This may be due to
the fact that differences in mean thermal expansion coefficient
between top coats and HC1 bond coat are higher than those having
Ni-Al bond coats.
Alumina is mainly used as wear resistant coating on steel.
Although zirconia or zircon is used as a thermal barrier coating
on metals, but they have acceptable wear resistance properties
too. Wear is expressed in terms of cumulative weight loss as a
function of time^iar-thiD-Uwi/fi. Results of the wear testing of
both alumina and ZrO + SiO are shown in figure 5. It is quite
clear from the plot (figure 5) that Indal-alumina has offered a
superior wear resistance compared to Metco-alumina.
Figure 5 shows the wear behaviour of the as-sprayed PDZ coating
o
as well as coating subjected to heat treatment at 9(30 C for
different span of time. It is quite interesting to note from this
figure that the heat treated coating has better wear resistance
than the as—sprayed one. Definitely heat treatment plays a major
role in changing the residual stresses of the coating. An
extensive m—ray diffrsctometry work has been carried out to
characterise these coatings. It is seen that x-ray diffraction
pattern of as received PDZ powder shows mostly monocllnic
zirconia phase and zircon phase present in the powder.
7
The wear behaviour of calcia stabilised zircon coatings, both as sprayed and annealed
are shown in figure5(c). With calcia, not only thermal stabilisation but also chemical
stabilisation of the high temperature zirconia phases are achieved. These stabilisations
cause significant improvement in the wear resistance of the as sprayed coatings. Both
cubic and monoclinic zirconia phases are present in the calcia stabilised zircon coating.
Silica is present in the form of separate amorphous phase. Cubic zirconia is present
owing to both chemical and thermal stabilisation. Calcia causes chemical stabilisation of
the high temperature cubic phase whereas rapid solidification is responsible for the
thermal stabilisation of the high temperature phase. It is important to note that more
cubic zirconia peaks are present and less monoclinic zirconia peaks are present in the
zircon - 20 wt% calcia as sprayed coating than in the case of as - sprayed PDZ coating,
as observed in the x-ray diffraction pattern. Annealing at 900°C for one hour does not
cause any significant change in the x-ray diffraction pattern of the zircon - 20 wt% calcia
coating. With extended annealing upto six and a half hours, the structure remains
reasonably intact. Some decomposition of high temperature zirconia phases to low
temperature monoclinic zirconia phase may take place along with reassociation of some
zirconia and silica to form zirconium-silicate (ZrSi04). However, owing to the chemical
stabilisation by calcia, these coatings undergo less transformation of high temperature
zirconia phases to the low temperature zirconia phase. The role of annealing is mainly
to eliminate residual thermal stresses which in turn increases the wear resistance of
calcia stabilised zircon coating by a significant amount.
GRINDINDABILITY OF PLASMA SPRAYED CERAMIC COATINGS
The responses that have been taken into account for grindability of coatings are ground
chip morphology, ground surface morphology, and grinding forces. Another important
response was the specific grinding energy, i.e., energy spent in grinding unit volume of a
material. The magnitude of specific grinding energy can be calculated from other
responses and parameters.
It has been observed that during grinding, blocky fractured chips are produced. It shows
that the material removal has taken place through fracture process. On the otherhand
the SEM observation of ground surface shows that some shearing has also taken place
along with quasi-cleavage fracture.
The grinding forces (both normal and tangential components) are plotted in figures 6(a)
and 6(b), respectively. PDZ shows the highest forces required for grinding among all the
coatings. Possibly it has the highest cohesion amongst all the coating and hence offers
the highest resistance to grinding.
The specific energy lies in the range of 10-35 J/mm3 as shown in figure 6(c). Similar
values has been obtained in grinding sintered ceramics as reported in the literature.
The comparative wear characteristics of alumina-zircon and zircon
top coat with Ni-Al bond coat and the one with high carbon iron
bond coat is depicted in the graph in Figure 7. It shows that the
zircon-alumina top coat with Ni-Al bond coat far outweighs the
o t he r c:omb i n a t i on s .
Thermal fatigue resistance obtained is also promising, the
failure of the coating is due to the peel off from the oxzidised
layer of substrate and the bond coat and not due to the bond
coat-top coat peel off.
The grindability study reveals that the grinding forces and
specific energy of grinding is within an acceptable range, thus
(suggesting that the coatings have a good grindability. ».________
We Claim:
1. A process for coating a substrate comprising the steps of:
- grinding a coating material such as zircon to obtain a powder for
coating;
- subjecting a substrate to cleaning in a manner such as herein
described to obtain the substrate with a cleaned surface;
- applying a Ni-5wt% Al bond coat on the cleaned surface by a
known process to obtain a bond-coated surface;
- followed by subjecting the bond coated substrate with the
bondcoat to plasma spraying with the powder for coating.
2. The process as claimed in claim 1 wherein coating material used is zircon
and optionally a compound selected from alumina and calcia.
3. The process as claimed in claim 1 wherein the step of grinding comprises
ball milling the coating material in an organic solvent such as toluene.
4. The process as claimed in claim 3 wherein the step of bail milling is
carried out for two and a half hours to five hours, preferably for three
hours.
5. The process as claimed in claim 3 wherein the weight ratio of the ball to
powder is about 10:1.
6. The process as claimed in claim 1 wherein the particle size of the powder
used for coating is in the range of 45 µm to 100 µm.
7. The process as claimed in claim 1 wherein the step of cleaning comprises
a first step of ultrasonic cleaning in an organic solvent followed by rinsing
in an alcohol solvent, followed by a second step of ultrasonic cleaning in
an organic solvent such as trichoroethylene,
8. The process as claimed in claim 7 wherein said first and second steps of
ultrasonic cleaning in an organic solvent such as trichloroethylene.
9. The process as claimed in claim 7 wherein said step of rinsing comprises
rinsing with an alcohol such as methanol, ethanol, isopropanol.
10. The process as claimed in claim 7 wherein said first and second steps of
ultrasonic cleaning are carried out over a period ranging from 3 to 10
mins..
11. The process as claimed in claim 1 wherein for the step of plasma
spraying, a primary gas such as nitrogen and a secondary gas such as
hydrogen is used.
12. The process as claimed in claim 2 wherein for the coating, zircon and
alumina are present in a ratio of approximately 2:3 mol%.
13. The process as claimed in claim 2 wherein zircon and calcia are present in
a proportion of 20 wt% calcia per unit weight of zircon.
14. A coating for use in plasma spraying of substrates comprising zircon and
alumina in a ratio of about 2:3 mol%.
IS. A coating for use in plasma spraying of substrates comprising zircon and
cakia in a proportion of 20 wt% cakia per unit weight of zircon.

This invention relates to a process for coating a substrate
comprising the steps of :
grinding a coating material such as zircon to obtain a powder for
coating;
subjecting a substrate to cleaning to obtain the substrate with a
cleaned surface;
applying a Ni-Al bond coat on the cleaned surface by a known
process to obtain a bond-coated surface;
followed by subjecting the bond coated substrate with the bond
coat to plasma spraying with the powder for coating.