24-11-2012, 12:28 PM
A study of surface texturing of carbon steel by photochemical machining
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ABSTRACT
Photochemical machining (PCM) was utilized to fabricate microtextures on carbon steel surfaces in this
study. A series of experiments were carried out in the two stages of this process, photolithography and wet
chemical etching. In the first stage, the influences of photolithography parameters, including spin coating
speed, exposure time and development time, on the patterning of photoresist films were investigated, and
the optimum process parameters were found. In the second stage, through a trial and error approach,
it has been found that the mixture solution of HNO3:H3PO4:H2O = 8.5%:59.5%:32% (mass percentage)
is a suitable etchant for the wet chemical etching of carbon steel. Based on the optimum results, the
microtextures of circles and right triangles with different sizes were fabricated on the end faces of carbon
steel discs. The variation of the end face profiles and etching depths of the microtextures with the etching
time was studied. A prediction model of the geometry of the fabricated microtextures was proposed. Its
accuracy was verified by comparison with additional experiment results, and its application scope was
also discussed.
Introduction
Surface texturing has been viewed as one of the effective surface
engineering technologies to significantly improve tribological
performance of mechanical parts. In the past two decades, with
the advances in precision manufacturing technologies, surface
texturing has been successfully applied to many products, such
as mechanical seals (Etsion and Halperin, 2002), thrust bearings
(Brizmer et al., 2003) and piston rings (Ryk et al., 2002).
Up to now, many practical surface texturing methods have been
developed, among which laser surface texturing (LST) is the most
widely used one. It provides flexible control of the shape and size
of a texture cell, and is able to process many kinds of materials.
However, adverse heat affected zones, microcracks and redundant
bulges are the shortages of the LST process. Etsion (2005) reviewed
the efforts being made world wide on LST. On the other hand,
many other micro/nanomanufacturing technologies have also been
applied to surface texturing. Wang et al. (2006) used lithography
and reactive ion etching (RIE) processes to fabricate microtextures
on the end face of a silicon carbide (SiC) disc, and obtained regular
dimple patterns with smooth etched bottoms.
Experimental details
Experiments were carried out on the discs made of mild carbon
steel (ASTM 1020), which were carburized and quenched to the
microhardness HV779. The diameter and thickness of a disc were
24 mm and 8 mm, respectively, and its end faces were lapped to the
surface roughness Ra = 0.11 m and flatness no more than 2 m.
The process of texture fabrication by PCM contained eight main
steps, as shown schematically in Fig. 1, and the whole steps were
operated in a Class 1000 clean room.
Results and discussion
Pattern resolution of the film-pasted photomask
Because the pattern resolution of the film-pasted photomask
was around 20 m, the circle profile with the designed diameter of
20 m deformed partly, as shown in Fig. 3(a). For the larger circles,
the shapes and sizes were almost the same as designed. The right
triangle shape with the designed hypotenuse length of 20 m deviated
from the designed one thoroughly, as shown in Fig. 3(b). For
the larger right triangles, the designed sharp corners were substituted
by fillets, as shown in Fig. 3©. It was found that the fillet radii
of a right triangle at the three corners were independent on the triangle
size, and their average values were 4.0 m (30◦), 5.4 m (60◦)
and 6.4 m (90◦), respectively. If the sharp corners of a right triangle
dimple fabricated by PCM are demanded, then special corner
compensating structures should be designed for the right triangle
pattern on the photomask. This technique has been widely used in
fabrication of various shapes of MEMS structures (Pal et al., 2009).
Effects of exposure time on photoresist patterns
Fig. 5 shows the optical microscope images of the photoresist
patterns of right triangles and circles under different exposure time
periods, where the development time was 10 s and the photoresist
patterns have been developed completely. It can be seen that for
the right triangle, three fillets at the three corners have been reproduced
from the photomask. The increase of the three fillet radii and
the size of the right triangle when the exposure time increases from
4 s to 12 s is much more significant than that for the exposure time
from 12 s to 30 s. For the circle, the exposure time only influences
its dimension. The circle diameters corresponding to the exposure
time of 4, 12 and 30 s are 48, 52 and 52 m, respectively.
Because of the geometric errors of the disc end face, such as
flatness (<2 m) and surface roughness (Ra = 0.11 m), there was a
small gap between the photomask and the photoresist film surface
during the photolithography process, even though the hard-contact
exposure had been used. According to the simulated light intensity
distribution on the photoresist film at different gap distances
(Cui, 2005), it is found that the light intensity is non-uniform
and decreases sharply at the edge of a photomask pattern until it
becomes zero at a position a few microns or tens of microns away
from the edge (depending on the gap distance).
Conclusions
The process of PCM was used to fabricate microtextures on the
end faces of ASTM 1020 steel discs. The influences of the photolithography
parameters on the patterned photoresist films were
investigated, and the effects of the etching time on the etched
topography of circle and right triangle dimples were studied.