Robert Ohlsson

The accuracy of fast 3D topography measurements

Abstract One important factor in surface engineering is the evaluation of the surface topography, i.e. measurement and analysis. These measurements of the surface roughness are normally done by stylus instruments and 2D profiles due to available standards. 3D surface topography evaluation is, however, in some cases necessary and frequently used, a standard is under development, and newly developed measurement instruments are used. Specially interferometric instruments are of interest here since measuring time is critical and their accuracy and comparability to stylus instruments must be evaluated. Therefore, in this study a number of engineering surfaces have been measured and evaluated: plateau-hones cylinder liners for engines, polished steel roller for bearings, grounded and honed gear surfaces from gear boxes, and steel sheet surfaces used in car bodies. By the use of a precise relocation technique, an exact comparison was possible between an interferometric instrument (Wyko RST Plus) and a 3D stylus instrument (Somicronic 3D). To be able to judge and analyze differences between the instruments an Atomic Force Microscope (AFM) was used. The AFM is extremely accurate for this type of surfaces, but limited in its measuring range. The result shows a very good agreement between the instruments with deviations of approximately 5–20% depending on the parameter evaluated. The stylus in general gives lower values. The results from the AFM are generally found between the parameters of these instruments, and for soft surfaces, such as sheets or smooth surfaces, the interference instrument has a slightly better agreement to the AFM. This is due to the stylus tip geometry and the relatively high contact pressure having difficulties measuring small features and also damaging softer surfaces. The result from an interferometric measurement occasionally shows optically introduced artifacts caused by local surface slopes which exaggerate the topography. By knowing the degree of influence from these peculiarities it is still possible to use fast interferometric instruments and to estimate its deviation from what would have been the result with a stylus instrument.

Wear of cylinder bore microtopography

Atomic force microscopy (AFM) reveals additional 3D topography information on tribology surfaces previously measured and evaluated by conventional 2D stylus technologies. This paper deals with the implications of the more detailed topographical information scanned from cast iron automotive cylinder liners. Worn and unworn surfaces measured both by AFM and stylus techniques were compared visually and quantitatively using an effective relocation technique. Quantitative comparison was made of 3D and 2D surface parameters, such as root mean square roughness, and slope, significant for the tribological behaviour of the surfaces. The extra surface features found by the AFM measurements (e.g. steeper slopes and more peaks and valleys) significantly change the numerical values of the roughness parameters, and this scale-dependent difference, when compared with conventional stylus-measured parameters, points to the possibilities of deepening the understanding of cylinder liner lubrication in the light of more finely detailed measurements.

Accuracy of replica materials when measuring engineering surfaces

Abstract This paper presents a comprehensive study of three different materials used for surface roughness replication on five different types of machined surfaces. Steel sheets (electron beam texturing and shot blasted texturing), a cylinder liner, a crankshaft and a face-ground surface. The three different replica materials used were Araldite , Microset and Technovit. The surfaces were measured with the non-contact WYKO interference instrument, zoomed to relocate the same area on the original and on the replicas. Finally, three-dimensional surface parameters were calculated to compare each replica with the original surface. The results show that all materials are possible to use in order to measure and calculate three-dimensional surface parameters, but they have some peculiarities. The three replicas all show good accuracy on all surfaces, with deviations S z values that are >10% too high due to a small number of pores in these materials. Replicas in the Microset material in general show somewhat lower parameter values than the original surface. All replicas showed some problems on the fine-ground crankshaft with an enlargement of the fine structure, resulting in parameters ca 20% too large. The deviation of surface parameters on repeated measuring on replicas is considerably larger than on repeated measuring of the original surfaces.

Nano metrology of cylinder bore wear

Abstract Cylinder bores are multi-process surfaces whose roughness is difficult to characterise for tribological purposes by conventional methods. Statistical approaches may be used to compute asperity densities, summit curvatures and so on, but suffer from the usual disadvantage of tending to infinite values in the absence of a short-wavelength cutoff. A useful advance in tribological roughness assessment would be to find a means of establishing an appropriate scale of measurement. Using a form of the plasticity index corrected for anisotropy, a short-wavelength limit λ p is derived below which asperities will not take part in long-term tribological interactions. A general relationship is obtained between three dimensionless numbers, the short-wavelength limit λ p normalised by the topothesy Λ, the fractal dimension D and the material ratio (the ratio of the Hertzian elastic modulus E′ to the hardness H). From this relationship, presented as a carpet plot, the appropriate scale of roughness measurement for any tribological investigation of a fractal surface may be determined. With a stylus instrument and an atomic force microscope, a number of cylinder bores were measured at locations of both high and medium wear before and after running in. By inspection of an ensemble of structure functions, it is shown that cylinder bore surfaces are multifractal, with a transition point (the so-called “corner frequency”) at about 20 μn, corresponding to the average size of a honing grit. Below this length the surfaces are self-similar fractals down to the limits of AFM resolution. The short wavelength limit using the above formulation appears to be about 40 nm, weI1 below the range of instruments usually employed to measure tribological surface roughness.

A Novel Approach to Reduction of Frictional Losses in a Heavy-Duty Diesel Engine by Reducing the Hydrodynamic Frictional Losses

An important parameter in the reduction of fuel consumption of heavy-duty diesel engines is the Power Cylinder Unit (PCU); the PCU is the single largest contributor to engine frictional losses. Much attention, from both academia and industry, has been paid to reducing the frictional losses of the PCU in the boundary and mixed lubrication regime. However, previous studies have shown that a large portion of frictional losses in the PCU occur in the hydrodynamic lubrication regime. A novel texturing design with large types of surface features was experimentally analyzed using a tribometer setup. The experimental result shows a significant reduction of friction loss for the textured surfaces. Additionally, the textured surface did not exhibit wear. On the contrary, it was shown that the textured surfaces exhibited a smaller amount of abrasive scratches on the plateaus (compared to the reference plateau honed surface) due to entrapment of wear particles within the textures. The decrease in hydrodynamic friction for the textured surfaces relates to the relative increase of oil film thickness within the textures. A tentative example is given which describes a method of decreasing hydrodynamic frictional losses in the full-scale application.