In Mechanical Finishing, All That Glistens Is … Or Is It?
Surface finish types for commercially supplied stainless steel sheet are detailed in various standards. ASTM A480-12 and EN10088-2 are two; BS 1449-2 (1983) is still available, although no longer active. These standards are very similar in that they define eight grades of surface finish for stainless steel. Grade 7 is “buff polished,” while the highest polish—the so-called mirror polish—is designated Grade 8
Introduction: Polished stainless steels
Ever since the discovery in 1912, of the family of alloys collectively known as stainless steels, many of their applications have required that they be provided with a polished finish. Cutlery, domestic appliances and tableware are but a few such applications. For some architectural applications, such as handrails, a relatively low degree of polish is required. Where aesthetics come into play, a higher degree of polished finish is required, as with tableware. Such finishes are usually created using electropolishing. However the ultimate polished finish on stainless steel is the so-called mirror-finish achieved using mechanical polishing. The types of surface finish on commercially supplied stainless steel sheet are laid down in various Standards. ASTM A480-12, EN10088-2 are two such standards. BS 1449-2 (1983) is still available, though no longer active. These standards are very similar in that they define eight grades of surface finish for stainless steel. Grade #7 is “buff polished” while the highest polish, so-called Mirror Finish with which we are concerned here, is designated #8 (in the case of EN10088-2, it is designated 2P). These standards describe, in qualitative terms, how such a mirror finish is obtained, by mechanical polishing using progressively finer grades of abrasive and finally, a polishing compound. However these standards do not define the optical quality of surface finish in any quantitative way other than specifying “ a non-directional finish which is reflective and has good image clarity. The surface will be essentially free from grit lines due to the initial grinding stages”. We show here that, while such Standards are undoubtedly valuable, they can conceal considerable differences in nominally identical products from different suppliers.
Mirror-finish stainless steels
As mirror-finish stainless steels became commercially available, they were eagerly seized upon, initially by interior designers, for example in retail stores or hospitality venues, then by architects who welcomed their durability in external as well as internal settings. Last but not least, modern sculptors began to embody mirror-finish stainless in their structures or indeed to create an entire work of art using the material. Perhaps the most striking example is the “Cloud Gate” sculpture in Chicago as shown in Figure 1. Created by sculptor Amish Kapoor in 2004-2006, this massive piece dominates the AT & T Plaza and is “functional” in that - as a convex mirror – it reveals a view of the city and the lake shore. It measures 10 x 20 x 13 m and weighs around 100 tonnes. A lengthy Wikipedia entry provides some details of its polishing and maintenance (1)
Production of Mirror-Finish Stainless Steel & Standards
Mirror-finish stainless is produced by mechanical polishing. Details will not be described here, but a succinct description is given by Lehnen (2) who notes that a final grit size of 430-400 should be used before polishing.
All that glistens ...
The work described here was triggered when the author, comparing several samples of #8 mirror-finish from different manufacturers, immediately recognised that, in optical terms, they were anything but identical. Even with only the naked eye, it was evident that – as mirrors – some were better than others. It was therefore decided to make a more rigorous comparison, using various instrumental techniques. Samples from three manufacturers formed the basis of the exercise, and these will here be designated as R, O and A.
Characterisation of Surfaces
Today, the characterisation of a surface in terms of its topography and optical properties, is a branch of science in itself. The term surface roughness now embodies a family of sub-definitions, Rx where different definitions represent peak-to-trough, root mean square or other definitions (3). Other, more mathematical concepts for surface characterisation are texture and curvature (4), the latter, somewhat misleadingly, is used to characterise a broadly planar surface in terms of the amplitude and spacing (frequency) of surface undulations.
Optical properties have long been characterised in terms of specular reflectance, also known as gloss. More recently, parameters such as DoI (Distinctness of Image) have been introduced. (5)
Instrumental Techniques for Surface Characterisation
Profilometry, in which a micron-sized stylus records surface topography as an electrical signal (which can then be processed) is one of the oldest methods for surface roughness measurement and still very widely used today. In the last decade or more, a range of optical, non-contact methods have been developed, the oldest being interferometry, More recently, a technique known as Phase Stepped Deflectometry has been developed in France, offering many advantages over older optical and other techniques. (for a fuller description see. 6)
Some general comments should be noted. A highly reflective surface may have random defects, such as scratches or pits. These can be too small to be observed with the naked eye. More important, however, is the question of isotropy (or anisotropy). As a result of mechanics of the polishing process, there are often systematic variations of property along the X- and Y-axes, and some such effects are revealed in the data reported below.
Using an Elcometer 7061 MarSurf PS1 profilometer, roughness data was obtained as shown in Table 1.