Characterization of WC-Co composite thermal spray powders and coatings

       

1.    INTRODUCTION

Thermal spray coatings have a wide range of applications,
for instance, by repairing machine parts damaged in service or by the
production of parts with high wear resistance. The wear resistance of a coating
does not depend only on the spray system used, but also on the characteristics
of the particular spray powder. Previous studies have demonstrated  that the use of recycled hardmetal powders in
the formation of detonation coatings leads to numerous problems. Hardmetal
powder particles sized from 32 to 40 µm in a detonation spray produce very
porous (4 to 5%) non-uniform coatings. Therefore, a new technology – mechanically
activated synthesis  – was used to
produce experi- 436 mental WC-based composite spray powders. The new
experimental powder increases the wear resistance of the coating . To guarantee
high abrasive wear resistance of a coating, it is necessary to optimize the
structure of the coating. Microstructure analysis gives combined
characterization of the morphology, elemental composition and crystallography of
the coating . Studies of the composite powder granule structure  show that no single universal method exists to
acquire all the information needed for the material structure characterization.
Thus the choice of the method depends on the researcher. However, sometimes the
chosen methods do not provide reliable information because of insufficient
experience. Inaccurate results typically occur, when computer-aided
measurements are relied on with no reservations. In fact, computer-aided
microstructural analysis can provide highly accurate information in a short
time. The article focuses on the analysis of the structure of the coating and
the use of the obtained data with the aim to improve abrasive wear resistance
of the coating.

2.    EXPERIMENTAL

  To analyse the structure of the particles and
coatings, cross-section polishes were made by a mechanical grinding-polishing
procedure. To analyse the structure and composition of a coating, cross-section
polishes were made by hot mounting. The best results for the powder were
obtained with a fluid (not viscous) cold mounting with an epoxy-based compound
that was mounted by help of the Buehler vacuum impregnation system. The
cross-section polishes of the coating were made by hot mounting. Due to the
high hardness of WCparticles, diamond grinding-polishing was used. The
microstructure of the powder granules and the coating was investigated by means
of the optical microscope Axiovert 25 and scanning electron microscope (SEM)
Jeol JSM-840A using backscattered electron imaging. Quantitative results of the
structure analysis were obtained by the image analysis systems Buehler Omnimet
Image Analysis System Version 5.40 (OM) and Image Pro 3.0. To describe the
microstructure of the grains and the shape of the powder granules, the
following parameters were used: roughness is defined as the ratio of the convex
perimeter to the perimeter (if there are no concave parts at the perimeter of
the particle, the roughness of the particle is 1.0); sphericity is defined as
4π × area/(perimeter squared) (if the shape of the particle is a perfect
circle, the sphericity of the particle is 1); aspect ratio refers to the
longest axis of the observed object, divided by the shortest axis of the latter
[6 ]. The phase composition study of investigated samples was carried out on a
Bruker D5005 X-ray diffractometer (XRD), using Cu Ka radiation at 40 kV and 40
mA. The range was 11 to 70° by step 0.040° and step time was 3 s.

3.    RESULTS

To produce composite powders with sub-micrometric WC
grains, it is necessary to analyse a sub-micrometric powder. Indirect methods
like BET or XRD can be used to measure the WC powder (from 20 to 500 nm). The
BET method enables us to find the size of the surface area and then calculate
the average particle size. The 438 XRD method provides a picture of
distribution (not precise, though) and an average value of the crystal size. In
the case of nano-size powders, the grain and the crystal are usually of the
same size. When treating nano-size particles, care should be taken not to
oxidize particles during measurements. For thermal spray, the granule size of
the powder should range from 20 to 45 µm to achieve high productivity of
spraying and to avoid oxidation processes [7 ]. It is much easier to determine
the size and size distribution of composite powder granules because of their
larger size. For these purposes, range, sieve and laser particle size analysis
can be used. Sieve and laser analysis may yield different results because of
the elongation of the granules (Fig. 1b). To characterize their elongation, the
aspect ratio AS [6 ] was used. The mean aspect ratio was 1.96, which means that
the longest diagonal of the particle is almost twice as long as the smallest
one. Because of such elongation, the granule size, measured by a laser
diffraction analyser, was larger than 45 µm (the largest by sieving). By the
laser analysis, 28% of the granules measured over 50 µm and 5% less than 20 µm
(smallest by sieving). Granules over 45 µm can go through the sieve because of
their elongated shape [8 ]. Granule size distribution is shown in Fig. 2.