450 Prakt. Met. Sonderband 30 (1999) 
Experimental 
Friction materials were prepared consisting of “permanent” and “varying” fractions, each being 50 vol.%. 
Permanent fraction consisted of 10 vol.% of brass, 10 vol.% of Sb,S;, 5 vol.% of artificial graphite and 25 vol.% 
of phenolic resin, modified by styrene-butadiene rubber (SBR) and hexamethylenetetramine. The varying 50 
vol.% contained different amounts of glass fibers (glassy fiber or glassy phase), Kevlar fibers, or cellulose fibers 
as will be shown later in temary diagrams. The used raw materials are included in table 1. 
| Material Trade name Company 
Brass fiber GBR - 2341 GMT Chicago 3 
Aramid fiber >. Kevlar | DuPont 
Glass fiber © Cerafiber HM25 K Thermal Ceramic Sch 
C fiber P25X Thornel 
Cellulose fiber Technocel 2004 Cellulose Fiber Company 
Artificial Graphite A - 625 Ashbury Graphite 
_Sb,S; S.P.Grade Anzon, Inc 
| Phenolic resin (dry) HRJ - 652 Schenectady International 
Table 1: Raw materials used for specimens preparation. 
The selected amount of constituents (approximately 250ml total volume) was mixed for 15 minutes. The 
macroscopic homogeneity of mixed lots was checked visually and if necessary further mixing was done. The 
final mixtures were loaded into cylindrical molds (inner diameter d=25.4mm) and pressed at a temperature of 
170°C for 10 minutes. The pressure varied depending on the constituents used in order to obtain the porosity of 
approximately 10%. The samples were unloaded removed from molds and subsequently post cured (cross- 
linking reaction takes place) at 170°C for 4 hours. The specimens for the FAST (15mm x 15mm x 5mm) were 
subsequently cut using a diamond saw, ground and dried (95 °C/30min). The FAST procedure was driven with 
constant friction force and two specimens of each type were tested. The friction coefficient was measured every 
five seconds and its final value was calculated as an average from the last 40 minutes of the test. Wear was 
calculated as a weight reduction during each FAST. 
Light microscopy (LM Nikon FX35), scanning electron microscopy (SEM Hitachi 2560) and transmission 
electron microscopy (TEM Hitachi H7100FA), linked with energy dispersive X-ray microanalysis (EDX Noran 
Voyager) were used for material characterization. The preparation of surface samples for TEM was based on 
replica technique. Coat of 15 weight % aqueous Polyvinyl Alcohol (PVA) was applied onto the inclined brake 
lining surfaces and allowed to dry. The PVA film was peeled from wom surface using either forceps or by 
supporting the film with a nylon backing fabric before peeling it from the friction surface. The resulting film was 
suspended in distilled demineralized water to dissolve PVA. The residua were collected onto Formvar holey 
films supported by 200 mesh copper grids and allowed to dry prior to investigating in TEM. 
Results and discussion 
The behavior of sample #151 containing 40 vol. % of Kevlar and 10 vol. % of glassy fiber when tested using 
FAST is shown in Fig. 1. Measured temperature increases in an exponential way with time, and the thickness of 
sample originally increases and starts to decrease in second half of the experiment. The coefficient of friction is 
relatively stable in case shown in Fig.1 and it does not depend significantly on temperature. Note that detected 
temperature shown in Fig. 1 differs from real surface temperature. Thickness variations shown in Fig. 1 are 
related to competitive phenomena of swelling and simultaneous wear of sample. In case of sample shown in Fig. 
1, the swelling process dominated in the first half of experiment and wear dominated in the second half. Since it 
is not appropriate to express wear from this thickness measurement, the weight data established before and after 
test were used to calculate wear.