Factors affecting wear resistance of alumina ceramic spray coating

       

 It
has been found that the hardness and fracture toughness of ceramic materials
are the key factors affecting the wear rate of spray coatings. Scholars in
various countries have carried out a lot of research work, which can be divided
into the following three aspects, ceramic grain size, second phase materials,
and coating tribological mechanism.

 

First,
they study grain size of alumina ceramic spray coating. Alumina ceramics can be
divided into single-phase ceramics and multiphase ceramics. In the research
field of the correlation between the grain size and the tribological properties
of ceramics, researchers mainly investigated the effect of the grain size of
the matrix phase (or the second phase) on the tribological properties of
ceramics.

The
results show that the wear rate of submicron ceramics spray coating is much
lower than that of micron ceramics in bovine serum albumin environment, and the
grain pull-out and grain boundary microcracks of submicron ceramics are
significantly less than those of coarse-grained alumina ceramics. Sedlacek and
other researchers studied the effect of different grain sizes of alumina matrix
on the wear properties. The grain size of alumina matrix varies from 0.8 μ m to
4 μ m, while the second phase SiC is nano size. The results show that the wear
resistance of alumina matrix with submicron size is better than that of nano
composite ceramics with micron size; there is no obvious relationship between
wear resistance and fracture toughness when the matrix with submicron size, but
the wear rate of alumina composite ceramics with micron size decreases with the
increase of hardness.

Obviously,
from the above examples, it can be seen that grain refinement can effectively
help to improve the structural uniformity of materials, including increasing
the density of materials and reducing the defects of materials. Wococarbide
adopts advanced equipment and manufacturing process. The powder produced by
wococarbide has fine grain and meets the requirements of good wear resistance.
However, previous experiments show that there is no obvious relationship
between wear resistance and fracture toughness when the matrix grains are in
submicron scale.

 

Second,
they study the Second phase materials of alumina ceramic spray coating.

In
the field of tribological properties of alumina composite ceramics, the
composition compounding, that is, the formation of composite materials by
adding various second phase, particles (or whiskers) is also the main way to
improve the tribological (or cutting) properties of alumina ceramic coating.
According to the different influence mechanism, it can be divided into the
second phase self lubrication mechanism, the second phase grain boundary
enhancement, the second phase tribochemical reaction mechanism and so on.

① Second
phase self lubrication mechanism

The
second phase solid lubricants such as graphite, CaF2, PbWO4, MoS2, BN and soft
metal can effectively reduce the friction coefficient of the coating, and
improve the tribological properties of the coating. Deng Jianxin et al
introduced 10% CaF2 solid lubricant into the alumina / TiC composite ceramic
matrix. Through cutting and friction experiments, it was found that CaF2 was
extruded and coated on the friction surface to form a self-lubricating film,
which can effectively prevent the adhesion between the material and the
friction pair, reduce the friction coefficient and play a self-lubricating
role.

②
Strengthening effect of second phase grain boundary

The
second phase (mainly particles and whiskers) is introduced into the alumina
ceramic matrix, and the residual stress is produced during the cooling process
of the material preparation by using the difference of thermal expansion
coefficient between the dispersed particles and the matrix material, so as to
achieve the effect of grain boundary strengthening. When the crack propagates
along the grain boundary, it is necessary to overcome not only the inherent
grain boundary energy of the matrix material, but also the additional energy
caused by the residual compressive stress On the other hand, because the
coefficient of thermal expansion of the second phase particles is less than
that of the matrix, volume effect will be produced during the cooling process,
and microcracks will be generated around the second phase particles, which will
induce the deflection of the cracks and consume more energy for crack
propagation; in addition, the second phase particles are generally spherical,
which makes the crack tip The end passivation can reduce the stress
concentration and prevent crack propagation, thus improving the tribological
properties of the coating.

③
Mechanism of second phase tribochemical reaction

The
tribochemical reaction mechanism of the second phase is that the second phase
doped in the alumina matrix reacts with the gas in the air (mainly oxygen) or
with the material of the counter wear pair when it rubs against the material of
the counter wear pair to produce lubricating film and reduce the friction
coefficient of the material, so as to improve the tribological properties of
the material.

Deng
Jianxin et al introduced TiB2 particles into the alumina ceramic matrix to
prepare the Al2O3 / TiB2 composite ceramic tool. When the tool was cutting with
45 # hardened steel, it was found that when the cutting speed was greater than
120m / min, that is, the cutting temperature was greater than 800 ℃, TiB2 in
the alumina / TiB2 composite ceramic tool reacted with oxygen to form TiO2 and
B2O3, because the elastic modulus and hardness of TiO2 were lower than that of
the matrix material Much, the shear strength is reduced, so the friction factor
of material coating decreases, the adhesion wear of the tool is reduced and the
wear resistance of the tool is improved.

 

Third,
they study the tribological mechanism of alumina ceramic spray coating.

The
application of alumina ceramic spray coating has different mechanisms. At
present, researchers have carried out extensive research on this aspect and obtained
some regular knowledge. Deng Jianxin and other researchers studied the
Tribological Mechanism of two kinds of High Temperature Self-lubricating
Ceramic Tool Materials: Al2O3 / TiB2 and Al2O3 / TiC / CaF2. The results show
that: in low speed dry cutting, the wear mechanism of Al2O3 / TiB2 ceramic tool
is adhesive wear and abrasive wear; while in high speed dry cutting, the wear
mechanism of tool is oxidation wear, and the reaction film formed by
tribochemical reaction on the tool surface plays a solid lubrication role,
which improves the wear resistance of the tool. With the increase of TiB2
content and cutting speed, the reaction mechanism increases The friction
reducing and anti-wear effect of the membrane is enhanced. Yao Shuqing and
other researchers studied the tribological properties and mechanism of three
kinds of Al2O3 based ceramic tool materials with different second phases. The
results show that the tribological properties of the ceramic tool materials are
related to the types of additives, and the order of their anti-wear properties
is al2o3gsicw, Al2O3 / Ti (C, N) The wear rate w increases with the increase of
E / h and decreases with the increase of KIC; the wear mechanism of Al2O3 / TiC
ceramic tool materials is mainly adhesive wear, while that of Al2O3 / Ti (C, n)
and Al2O3 / SiCw Ceramic Tool Materials is mainly abrasive wear.

 

In
conclusion, the friction and wear performance of the coating on the material
surface is a comprehensive performance, which is affected by many factors.
However, all aspects of research show that the refinement of grain size and
composition can effectively improve the strength and fracture toughness of
alumina ceramics, and then improve its tribological properties. If the research
in this field can continue to improve, it will effectively promote the further
application of alumina ceramic spray coating in various fields.