Breviary Technical Ceramics






10.5.4 Surfaces Definitions

The surfaces of ceramic materials in most cases do not display regular structures, but contain profile deviations which can be classified into roughness, waviness and shape. Important data for ceramic parts are:

Ra = mean roughness figure,
Rz = roughness depth,
Mr = material ratio, formerly : tp = bearing area.

The average roughness depth Rz is the mean value of 5 individual roughness depths obtained from consecutive separate samples of the roughness profile. The extrema within each of the sampling lengths are summed and the total range divided by the number of the profile's sampling lengths.

Figure 214: Calculation of the roughness depth Rz

The average roughness Ra is the computed average of all deviations of the roughness profile from the median line over the defined length. Ra theoretically corresponds to the distance between several lines when the peaks above the median and valleys below the median are converted into rectangles of equal size.

Figure 215: Calculation of the average roughness value Ra

The material ratio Mr (formerly: the bearing area tp) is the ratio of the load bearing surface area (at a given intersection line) to the entire area under consideration (over the reference path) in %.

Figure 216: Calculating the material ratio Mr

When using optical methods, Mr is defined as a ratio of areas.

Mr = (area down to a defined depth) / (total area)

Although the optical method is not standardised, it is often used since it is possible to measure very quickly and with high precision.

The above mentioned parameters are defined in DIN EN ISO 1302.

In order for the manufacturer and the customer to be able to compare surface measurement data, it is important that measurements are made under identical conditions. The specifications affect the tracing stylus instrument, the stylus, the stylus geometry, the measuring length, filters, instrument and instrument settings as well as the surface parameters used. Examples with fundamental conditions are listed in Table 34.

Table 34:
Example of the specification of measurement conditions

The following examples of surfaces (see p. 234 f.), material ratios (see p. 236) and surface profiles (see p. 237 f.) will provide an impression of the characteristics of ceramics surfaces.


<< back   home   next >>