Chapter 2 The Areal Field Parameters

The vast majority of surface texture parameters are the field parameters. The term field refers to the use of every data point measured in the evaluation area, as opposed to feature parameters that only take into account specific points, lines or areas. Field parameters allow the characterisation of surface heights, slopes, complexity, wavelength content, etc. They are defined in the specification standard ISO 25178 part 2. In this chapter the ISO areal field parameters will be presented along with limited guidance on their use. 2.1 A Short History of Areal Parameters The first areal surface texture measuring instruments were made available around 1987. Instrument manufacturers such as Zygo, Wyko and others started to provide parameters calculated on the data. Early areal parameters were often simple extrapolations of profile parameters (see Chap. 1 and Leach 2009 for a description of the profile parameters) and were either named simply Ra or sRa, and sometimes calculated using proprietary algorithms, leading to different parameter values on different instruments. The initial work by Stout et al. (1993a, b) leading to the ‘‘Blue Book’’ was covered in Chap. 1. During the same period, ISO technical committee TC 57 introduced a new concept, called Geometrical Product Specification and Verification (GPS for short) in order to unify specification standards dealing with dimensional analysis and surface texture. Then, in 1996 a new committee was created, TC 213, to develop GPS specification standards. One of the first actions of TC 213 was to entrust a group of researchers with the aim of developing the basis F. Blateyron (&) Digital Surf sarl, 16 rue Lavoisier, 25000 Besançon, France e-mail: fblateyron@digitalsurf.fr R. Leach (ed.), Characterisation of Areal Surface Texture, DOI: 10.1007/978-3-642-36458-7_2, Springer-Verlag Berlin Heidelberg 2013 15 of areal surface texture parameters. The historical perspective of the SURFSTAND, which led to the so-called ‘‘Green Book’’ (Blunt and Jiang 2003) was covered in Chap. 1. The SURFSTAND results were presented to ISO in January 2002 and officially transferred to TC 213, in order to start the standardisation process. In June 2002, TC 213 voted for the creation of a new working group, which was assigned the task of developing future international standards for areal surface texture. This working group (known as WG 16) met for the first time in January 2003. At the end of 2005, the ISO secretary allocated the number (ISO) 25178 to all areal surface texture standards, thereby giving the standards their official birth. These new standards were motivated by a shift that occurred in surface metrology: the shift of surface measurement from profile to areal and contact to non-contact (Jiang et al. 2007). The work allocated to WG 16, which would be implemented in the ISO 25178 standards, contained two parts: • to define the content of the areal surface texture standards, for specification and verification; and • to revise the existing profile standards to bring them into line with the new areal standards. The first part of ISO 25178, part 6 on classification of surface texture measuring instruments, was published in January 2010 (ISO 25178 part 6 2010). This was followed in June by a group of documents on instrument techniques: part 601 (ISO 25178 part 601 2010) and part 701 (ISO 25178 part 701 2010) on stylus profilometer and part 602 (ISO 25178 part 602 2010) on confocal chromatic instruments. The main document defining areal parameters, ISO 25178 part 2, was published in September 2012 (ISO 25178 part 2 2012). As highlighted in Chap. 1, more documents will follow in the forthcoming years. 2.2 Naming, Filtering and Calculation Conventions 2.2.1 Naming ISO 25178 part 2 defines symbols for surface texture parameters that have a prefix that is the capital letters S or V followed by one or several small letters that form the suffix. This suffix should not be written as a subscript but on the same line as the prefix. However, the final version of ISO 25178 part 2 uses subscripts in parameter names to comply with ISO, which states that symbols should only contain one letter and optional subscripts. The prefix S is used for the majority of parameters (for example, Sq, Sdr, Smr), the alternative being volume parameters that start with the letter V (for example, Vmp, Vvc). 16 F. Blateyron 2.2.2 Filtering Conditions Profile parameters are named after the type of surface profile from which they are calculated, for example R-parameters (Ra, Rsk, etc.) are calculated on the roughness profile, W-parameters (Wa, Wsk, etc.) are calculated on the waviness profile, while P-parameters are calculated on the primary profile (see ISO 4287 2000; Leach 2009). The type of surface is not taken into account with areal parameters. The Sa parameter can be calculated on a primary surface or a filtered surface—it will be called Sa in all cases. Therefore, it is important to provide the filtering conditions together with the parameter value.