3.5.1 Background
From the early discussion in section 3.3.2.1, the material properties of the stylus play an important role in the probing, especially on the force exerted in the measurement. Young’s modulus is one of the most important material properties to be considered when selecting the material for the stylus shaft and stylus tip.
For conventional CMMs, steel, ceramic and tungsten carbide are all materials regularly used for stylus shafts. However, for micro-probes, tungsten and tungsten carbide are the most popular materials for stylus shafts [31][39][12]. Tungsten and tungsten carbide are preferred because they have a high Young’s Modulus, favourable strength, toughness and hardness properties, and can be used reliably for manufacture using micro-machining processes[58][76].
97 The materials for the stylus tip sphere in micro-probes, both currently available in the market or in the research and development, are usually glass, silicon nitride, ruby, and sapphire [21][37][129]. However, for a stylus tip with diameter in the range of ten micrometres, only glass materials are available for manufacturing the stylus tip, owing to current limitations in manufacturing techniques. Another important point is that the selection material for the stylus tip sphere must also refer to material of the surface to be measured. The details of these material properties will be discussed in the next section.
3.5.2 Young’s modulus of material
Young’s modulus or elastic modulus is a measure of stiffness of an elastic material. The Young’s modulus of a material is defined as the ratio of stress to engineering strain during the linear elastic behaviour. For a device of given geometry, selecting a material with higher value of Young’s modulus will lead to the need for higher stress to generate a certain strain, that is it will require more load or force to deform its shape. Therefore, because a stylus system should be stiff (hard to distort), a high value of Young’s modulus is a characteristic needed in the stylus shaft [34]. However, the Young’s modulus of the stylus shaft material is not the only major factor that determines the stiffness of the stylus shaft. As discussed in section 3.4.4, the length and the diameter of the stylus shaft are strong factors that influence the stiffness of the stylus shaft. A key functional requirement is the ability of the stylus shaft to resist bending deflection during probing. Equation (3.2) in section 3.2.4 shows that bending stiffness is proportional to Young’s modulus and so a high modulus material should be selected for the stylus shaft in order to minimize its bending.
Elastic deflection, or distortion, at the contact point between the stylus tip sphere and the measured surface will also directly affect the measurement. This effect (modelled in equation 3.1 in section 3.2.4) depends on the scale of the contact and the effective Young’s modulus (reduced modulus) of the contact region. The reduced Young’s modulus between the stylus tip sphere and the measured surface can be calculated using equation 3.6 in section 3.3.2.2. It is mainly scale-independent, but might increase when the contact size approaches the grain size of the workpiece [26]. Selection for a higher reduced Young modulus can lead to larger allowable probing forces, implying generally that a material with a high Young’s modulus should be used for the tip sphere.
98 So, a material with high Young’s modulus is most suitable for the stylus shaft and stylus tip sphere because it is an important factor that favourably influences the stiffness at the stylus shaft, amount of bending at the stylus shaft, and also the allowable probing force.
3.5.3 Yield strength of the measured workpiece
For the micro CMM measurement at the micro- and nano-scales, it is important to ensure that the applied contact forces during probing do not cause plastic deformation on the surfaces of either the stylus tip or the measured workpiece. To avoid such plastic damage, the yield strengths of the materials for both surfaces are considered in the estimation of the allowable probing forces, as discussed in section 3.3.2.1. For the case where the material of measured workpiece is softer than the material of stylus tip, the yield strength of measured workpiece is the crucial one for the calculation of allowable probing force. However, if a measured workpiece of a harder material than the stylus tip, the yield strength of the stylus tip should be used in order to avoid plastic deformation and permanent damage occurring at the surface of the tip. Therefore, it is crucial to determine the material of the measured workpiece prior to selecting suitable styli and conducting the measurement task.
3.5.4 Adhesive and abrasive wear
The selection of material for the stylus tip sphere should also take note of the material of the surface intended to be measured. This is because an appropriate stylus tip material will avoid or minimise adhesive and abrasive wear when contacting the measured surface. This is clearly more critical in the scanning mode of probing. In essence, adhesive wear as the local pressure between asperities on two surfaces causes them to temporarily ‘weld’ together and subsequently break, pulling a small particle from one surface that might become free or might stay adhered. Material pick up, one of the important concerns in the present context, is the phenomenon where the stylus tip sphere will collect the particle or contamination on the measured surface during probing [2]. This transfer process of the particle may happen through the process of local adhesion at the microscopic level and break off during sliding. This adhesion of the particle to the stylus tip is permanent and cannot be removed by normal cleaning processes. Thus it will affect the shape of the stylus tip which will lead to the measurement error [121]. For instance, adhesive wear could occur when a stylus tip made
99 from ruby contacts a measured surface made from aluminium [121][2][21]. Adhesive wear also arises when the measured surface is repeatedly scanned by the stylus tip sphere as has been demonstrated previously in the literature [26].
Abrasive wear on the other hand occurs when a small hard particle is pushed into and removes a small piece of a softer counter-face. Free hard particles might be present in the contact region (perhaps including ones created by the oxidation of debris from adhesive wear) or a harder asperity on one surface might act similarly. Small particles might be removed from one or both surfaces during the probing process. Abrasive wear may occur due to several factors; some particle in the stylus act as abrasive, and atomic attraction between the materials may occur [121][21]. Stylus tips made from ruby are found to be good for measuring workpieces made from stainless steel and titanium but are not suitable for aluminium workpieces. Silicon nitride is the alternative material for stylus tips to measure aluminium workpiece. Zirconia and tungsten carbide are suitable materials for the stylus tips to measure workpieces made from cast iron [121].
Thus, the abrasive and adhesive wear between the stylus tip surface and the measured surface are factors needed to be considered when selecting the appropriate materials for stylus tip spheres. This factor is of greater importance for scanned probing, but even point probing in practice involves a small amount of sliding motion in the contact region.
3.5.5 Summary of design rules for material selection
(xxv) With reference to design rules number (V) and number (XIX), the material for stylus shaft with higher value of Young’s modulus will reduced the allowable probing force and will influence the stiffness of the stylus shaft.
(xxvi) The material of the measured workpiece should be considered in designing a measurement task and selecting the appropriate styli in order to avoid plastic deformation in the measurement. Generally, though, a stylus tip material has high Young’s modulus and higher strength will be preferable.
(xxvii) The selection material for stylus tip sphere should take account of the material of the surface intended to be measured to avoid or minimize the abrasive and adhesive wear.
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