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JUNE 28 WEDNESDAY MORNING
JS2-WeM-OR.1 TRIBOLOGICAL PROPERTIES OF SILICON NITRIDE CERAMICS
COATED WITH DLC AND DLC-Si AGAINST 316L STAINLESS STEEL J. R. Gomes. Departamento
de Engenharia Mecânica, CIICS, Universidade do Minho, 4800-058 Guimarães, Portugal.
J. M. Carrapichano. Departamento de Engenharia Mecânica, Instituto Superior de Engenharia de
Coimbra, 3040-228 Coimbra, Portugal. S. S. Camargo Jr., R. A. Simão, C. A. Achete. Departamento
de Engenharia Metalúrgica e de Materiais, Universidade Federal do Rio de Janeiro, Cx. Postal
68505, 21945-970 Rio de Janeiro, RJ, Brazil. R. F. Silva. Departamento de Engenharia Cerâmica e
do Vidro, CICECO, Universidade de Aveiro, 3810-193 Aveiro, Portugal.
Diamond-like carbon (DLC) is an adequate coating on a large variety of materials for tribological
purposes. Its intrinsic hardness, smoothness and solid lubricious capability are key properties that afford
an outstanding combination of low friction coefficient and high wear resistance. However, the
performance of a tribosystem is also determined by the type of counterface material, the environmental
conditions and the presence or not of interfacial media. For example, in the automotive parts
industry, the valve guides or the transmission gears are today coated with DLC. Another practical
example of contacts that may involve steel as the mating material of DLC are forming tools to manufacture
steel products. Plasma enhanced chemical vapour deposition (PECVD) of DLC-Si or pure
DLC coatings were performed, respectively, by conventional rf glow discharge from gaseous mixtures
of methane and silane or taking only pure methane. In this work, an engineering ceramic, silicon
nitride (Si 3 N 4 ), is used as DLC substrate aiming the minimization of adhesion problems, usually
found when some metallic substrates are employed. The ceramic substrates were placed onto the
cathode of the deposition system where the rf power was applied achieving self-bias voltages varying
from -200 to -800 V.
The tribological properties were assessed by pin (316L stainless steel)-on-disc (Si 3 N 4 ) experiments,
without lubricant, at room temperature, under ambient air and 50-60% relative humidity. The normal
applied load assumed the values of 10 N and 20 N. The sliding speeds varied from 0.2 m/s to 1.0
m/s. The characterisation of the disc and pin worn surfaces was carried out by scanning electron microscopy
provided with energy dispersive X-ray spectrometry (SEM/EDS) and atomic force microscopy
(AFM). A first set of experiments was carried out in order to compare the tribological behaviour
of DLC-Si and pure DLC coatings on Si 3 N 4 . An early partial delamination of the DLC-Si coatings
was observed, although leading to a steady-state friction regime with friction coefficients
around 0.14, a low value in unlubricated conditions. This favourable frictional response is due to the
wear-induced surface graphitisation and to the role of self-lubricating layers of adhered debris in
both the sliding surfaces. The stainless steel contact surface was extensively covered by a Si, C and
O rich tribolayer, promoting a third-body protection to the steel pin. An improved tribological response
was obtained with pure DLC coated Si 3 N 4 discs sliding against the stainless steel pins. This
system almost instantaneously attains the steady-state friction regime, keeping the DLC film integrity,
contrarily to the DLC-Si damaging, which leads to noisy friction curves coming from cyclic
frictional events. The preservation of the coating demonstrates an adequate adhesion for tribological
purposes up to 20 N of applied load. The wear coefficient of the pure DLC coated Si 3 N 4 disc was, at
least, one order of magnitude lower than that of the DLC-Si coated one, always below 10 -6 mm 3 N -
1 m -1 . The set of tribological experiments of pure DLC coatings obtained under distinct self-bias application
during the PECVD process revealed that the wear coefficient values obtained for the –200V
biased coating are slightly better than those presented by the –800V biased one. However, with respect
to the friction coefficient values, a trend to lower values occurs for the –800V biased films.
Additionally, the typical friction pattern for low biased grown coating is much more unstable than
the very smooth one of the –800V biased film.
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