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The fabrication of many micro electro mechanical systems (MEMS) is mainly based on silicon and its compounds and, for moving structures such as micromotors, tribological behaviour plays a key role in the performance. In this paper the wear of MEMS-compatible materials has been investigated for a range of contact areas and contact forces typical of MEMS. Special test specimens incorporating a range of micromachined micro structures on their top surfaces have been fabricated in order to simulate those conditions. These single crystal silicon (SCS) micro structures were coated with a range of materials used in MEMS; diamond-like carbon (DLC), silicon nitride (Si3N4), silicon dioxide (SiO2), and doped and undoped polysilicon. A specimen-on-disc arrangement (a development of the macroscopic pin-on-disc) was used for the wear experiments and dead weight loading was applied using micro loads specially calibrated for this purpose. The results show that the wear rates of DLC and SCS sliding on DLC decrease with increasing sliding distance whereas Si3N4 and SiO2 showed approximately linear wear behaviour. The effect of contact morphology and contact pressure was investigated for doped polysilicon sliding on DLC and for doped and undoped polysilicon sliding on Si3N4. The results can be attributed to the differing mechanical and chemical properties of the materials leading to wear mechanisms ranging from asperity fracture to asperity deformation.

The variations of acoustic-noise power radiated from electric machines, having varying degrees of eccentricity of the rotor in the stator bore, were studied using a specially constructed rig in an anechoic chamber. A series of noise components, which were found in expsriments to vary considerably with rotor eccentricity, were identified analytically from the expressions for the mode numbers and frequencies of the magneticforce waves, taking into account the stator and rotor slotting, eccentricity and iron saturation. A method is presented for calculating the variation of important noise components with eccentricity. The effects on the noise power of different numbers of stator parallel paths and of different ways of connecting them were investigated using specially wound machines. Information is given concerning the choice of parallel paths and connections for noise reduction. List of principal symbolsA rd = amplitude of radial displacement, m a = outer radius of stator, m B = flux density, T b = halflength of the cylindrical part of machine, m c = speed of sound, m/s £" = modified Young's modulus, i.e. E/(l -y 2 ) N/m 2 , F = m.m.f., A / = frequency, Hz g = mean air-gap length, m g' = effective mean air-gap length, m h = parameter in sound power expression / = any integer J,(JC) = Bessel function of the first kind K,(x) = modified Bessel function k = order of m.m.f. space harmonic (Section 3.1) k = wave number, i.e. o»/c (Section 3.3) m = mode of vibration P = sound power, W p = number of pole pairs q -order of current time harmonic r 0 = radius of neutral layer, m r c = radius of stator core at the bottom of slot, m t = time, s Y/(x) = Bessel function of the second kind Z = number of slots z = axial distance (z -0 at centre of machine) a = angle of skew, rad 8 -logarithmic decrement e = relative eccentricity (infinitesimal quantity in eqn. 29) e' = effective relative eccentricity 6 = angle, rad A = permeance, Wb/A y -Poisson's ratio [Mo = permeability of free space, H/m p 0 = density of air, kg/m 3 a ra = radial force per unit area, N/m 2 = angular position, rad ifi -phase angle of m.m.f. wave co = angular frequency, rad/s Subscripts 1 supply c core dy dynamic ec eccentricity

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