Influence of abrasive particle size and contact stress on the wear rate of dental restorative materials

Alan Harrison and Graham E. Moores Department of Restorative Dentistry, University of Leeds, Leeds, UK.

Harrison A, Moores GE. Influence of abrasive particle size and contact stress on the wear rate of dental restorative materials. Dental Materials 1985; 1: 15-18.

Abstract. - Abrasive wear tests have been performed on 4 restorative materials using different grades of silicon carbide paper as the counterface. Also the variation in the wear rate with contact stress was investigated. A linear relationship between wear rate and contact stress for all 4 materials up to 1.3 MPa was established. Above this level of contact stress the performance of the conventional composite appeared to deteriorate. The magnitude of the wear rate of the 4 materials against the various grades of SiC paper differed one to another. The ranking orders of wear rates for the 4 materials were influenced by the choice of abrasive counterface.

Key words: abrasion, contact stress, amalgam, composite resin.

Dr. A. Harrison, Department of Restorative Dentistry, School of Dentistry, Clarendon Way, Leeds LS2 9LU, UK,

Accepted for publication 16 July 1984.

The requirements for a clinically suc- cessful restorative material are many and varied; however, with the devel- opment of composite resins for use as posterior restorations, improved wear resistance still has a high priority. Un- fortunately clinical evaluations of com- posites placed in stress-bearing areas of the mouth have provided disappointing results. Clinical trials are expensive and time-consuming and can investigate only small numbers of materials. For efficient and rapid screening of a wide range of materials it is necessary to use laboratory test procedures.

Although several wear tests have been described (1-6), the results have often been at variance with one an- other and in disagreement with clinical data. Many of the tests have used the classical pin on disc configuration where the test specimen has been posi- tioned on either the pin or opposing counterface. Where an abrasive paper has been used as the counterface 600 grit silicon carbide has often been ar- bitrarily chosen. This is considered by some workers to be more abrasive than anything encountered in the normal Western diet and testing of this nature has been criticised as being unrealistic. This may be true when different fa- milies of materials are being compared one with another but this type of test

may be valid when batch testing or when used as a basis for studying the mechanism of abrasive wear.

A linear relationship has been shown to exist between abrasive wear and the contact stress of homogeneous mate- rials (7, 8). The relationship between wear of dental restorative materials and the size of the abrasive particles is not clear, although there is some evi- dence to suggest that the wear rate in- creases with an increase in particle size (4, 11). Similar results have been shown for metals and polymers (9, 10).

The results of laboratory wear testing procedures for dental resto- rative materials are sometimes pres- ented in the form of rank orders. It is evident from the literature that these rank orders rarely agree with the findings of clinical trials (12-15). Bearing in mind the widely different compositions of restorative materials, it is probable that in vitro wear test rank orders will be influenced by the choice

of test parameters. The effect of the abrasive on the rank order of test spec- imens has been shown for both 2-body and 3-body wear processes (4, 11). The present study considers the effect of contact stress and size of abrasive par- ticle in 2-body wear. Amalgam, un- filled resin, conventional composite and microfilled composite are investi- gated using a variety of grades of SiC abrasive paper.

Material and methods Specimen preparation

Four dental restorative materials were chosen as being representative of the range of materials available and be- cause they have commonly been in- cluded in other in vivo and in vitro stu- dies. Details of the materials used are given in Table 1.

Specimens of the resin-based mate- rials were mixed according to the man- ufacturers' instructions and poly-

Table 1. Materials.

Material Manufacturer (Batch no.)

Concise 3M - Company (9M12/9M39) Isopast Vivadent (22-1C-1D-28) Sevriton Amalgamated Dental Trade Distributors Ltd. (BNWG 2WL/BNRK 40WD) Dispersalloy Johnson and Johnson Dental Products Co. (D30380 B 07816)

16 Harrison and Moores

Looding rod

Specimen~ ~ S p e c i m e n

Fig. 1. Specimen and holder.

merized in individual holders by means of a split mould. Af te r tr i turat ion the coherent amalgam mass was condensed into similar holders using the split mould. Hence cylindrical specimens 3.5 mm in diameter and 3.0 mm long were produced, each in its own stainless steel holder (see Fig. 1). The specimen holders had an internal thread to facil- itate at tachment to the end of the wear machine loading rods. Five specimens of each material were made and stored in distilled water at 37~ for 21 days prior to the wear experiments.

Wear test method

Abrasive wear exper iments were carried out on a previously described wear machine (16) which was designed to test materials under condit ions sim- ulating the loads, sliding distance and contact times encountered in the human masticatory cycle.

Af te r a brief wear period (against the grade of silicon carbide paper to be used in the experiment) to ensure that the wearing surface of the specimen was flat, the length of each specimen and its holder was measured in trip- licate using a bench micrometer ac- curate to +1 ~tm.

Each group of specimens was mounted in turn on the lower end of the loading rods and abraded on the machine against silicon carbide paper under flowing water at 35~ The tests were cont inued until at least 100 ~tm

Table 2. Ranking order of wear rates for the 4 materials.

Material SiC grit number

120 180 220 400 600 800 1 0 0 0 1200

Amalgam 1 1 1 1 2 2 2 2 Conventional composite ! ~ 2 2 1 Microfilled "composite" 3 3 3 3 3 Unfilled resin 4 4 4 4 4 4 4

Table 3. Approximate values of the 50% median particle size for the SiC grits and conventional composite filler.

Material Median particle size ~tm

Conventional composite filler 13

SiC 120 120 180 80

" 220 60 400 35 600 26

" 800 22 " 1000 18 " 1200 15

SiC grits particle size distribution were kindly supplied by English Abrasives Ltd,, London, UK.

had been removed from each specimen and the silicon carbide paper was re- newed with sufficient regularity to ensure that no loss of abrasive effi- ciency had occurred. Suitable intervals between changes of paper had been de- termined from previous experiments .

The method given above was repea- ted using 8 different grits of paper ranging from 120 to 1200 (see Table 3). In the first series the specimens were loaded to produce a stress of 0.25 MPa (Nmm 2) normal to the wear face and the loads increased in steps up to a maximum of 2.1 MPa.

At the end of each exper iment the length of each specimen and holder was measured in triplicate and the loss of material determined. The mean wear rate of the group was calculated f rom the wear of the specimens within the group.

Filler particle size measurement

Specimens of the convent ional compo- site were ashed by the me thod de- scribed (17) and SEM photographs were taken of the remaining quartz filler. The particle size distribution was determined by image analysis of the photographs.

Results The variation of the wear rate of the 4 dental restorative materials with contact stress is given in Fig. 2. The error bars shown on this figure are _+2 standard errors and the solid lines are least squares fits to the data. The ordi- nates are in the directly measured units of ~tm min z.

The results of experiments to deter- mine the variat ion in the wear rate of the 4 dental materials with silicon car- bide grit number are given in Figs. 3-6. The results shown in these figures are for contact stresses of 0.25 MPa (hatched bars) and 0.45 MPa (plain bars). The bars are the mean values and the error bars are _+2 standard er- rors.

The ranking orders of wear rates for the 4 materials, when abraded by various grades of silicon carbide paper ,

50 . . ~

~,0~ e , /

f "

10

oI~ 0% I.'~ ,,; 2'-0 2!, Contc'/ct stress {MPO ]

Fig. 2. Variation in wear rate with contact stress for 2 grit sizes of abrasive. (a) Unfilled resin, 600 grit SiC (b) Unfilled resin, 1000 grit SiC (c) Microfilled "composite", 600 grit SiC. (d) Microfilled "composite", 1000 grit SiC (e) Conventional composite, 1000 grit SiC (f) Conventional composite, 600 grit SiC. (g) Amalgam, 600 grit SiC. (h) Amalgam, 1000 grit SiC.

Abrasive particle size and contact stress 17

40

30

2o

L. b

i i l~o i~o ~ ~ =o ~o t ~ ' 1200

S i C g r i t no.

Fig. 3. Variation in the wear rate of the amalgam specimens with SiC grit number. Hatched bars - 0.25 MPa, plain bars - 0.45 MPa.

are given in Table 2. Vertical bars join results which are not significantly dif- ferent at the 90% confidence level.

The approximate median particle size of the silicon carbide particles is given in Table 3, together with the ap- proximate median particle size of the filler in the convent ional composite.

Discussion

It is difficult to assess, from the available l i terature, the contact stress likely to be encountered in normal che- wing, since the area over which the force acts is usually unspecified. A number of wear tests have been perfor- med at a contact stress of the order of i

40

30

~, 20

I'd() I~0 ~0 400 600 I ~ @ I~0 S i C grt~ no.

Fig. 4. Variation in the wear rate of the un- filled resin specimens with SiC grit number. Hatched bars - 0.25 MPa, plain bars - 0.45 MPa.

40

3O T" .s

ZO

120 180 1200

S i C grit n o

Fig. 5. Variation in the wear rate of the mi- crofilled "composite" specimens with SiC grit number. Hatched bars - 0.25 MPa, plain bars - 0.45 MPa.

MPa (2, 3, 18). Catastrophic failure of a conventional composi te material has been repor ted (2) when the contact stress was approximately 1.4 MPa.

Fig. 2 shows the variation in wear rate with contact stress for the 4 mate- rials, each against 2 grades of abrasive paper. Be tween 0.25 MPa and 1.3 MPa there is a linear relationship between wear rate and contact stress for the un- filled resin, the microfil led "composi te" and the convent ional composite. The linear relat ionship between wear rate and contact stress extended up to 2.1 MPa in the case of amalgam. In the linear range, correlat ion coefficients greater than 0.985 were found for the 4 materials. At 2.1 MPa contact stress

40

3O

i i i i i

S i C gr i t no.

Fig. 6. Variation in the wear rate of the con- ventional composite specimens with SiC grit number. Hatched bars - 0.25 MPa, plain bars - 0.45 MPa.

there is no evidence to suggest drastic changes in behavior of any of the mate- rials. However , above 1.3 MPa the conventional composi te resin shows an increase in the rate of change of wear rate with contact stress whereas the other 2 polymer-based materials show the reverse trend. The gradients of the linear plots shown on Fig. 2 are dif- ferent for the 4 materials and con- sequently the wear rates relative to amalgam alter with the change in contact stress. When worn against 600 grit SiC paper the ratio of the wear rates for amalgam: conventional com- posite: microfil led "composi te" : un- filled resin are 1: 0.75: 2 .3 :3 .3 at 0.25 MPa, 1: 1.6: 4 .4 :5 .4 at 1.3 MPa and 1: 2.4: 3 .6 :4 .3 at 2.1 MPa. This illustrates the importance when reporting wear rates of defining both the area of contact of the specimen and the applied load.

Al though the wear rates of the amalgam, microfil led "composi te" and unfilled resin differ in magnitude, the variation in the wear rates with SiC grit number fol low a similar trend; for the amalgam and the unfilled resin the sim- ilarities are marked. At a contact stress of 0.25 MPa the maximum wear rates occur in the region 220 to 400 grit and at the higher contact stress of 0.45 MPa the peak is at 180 grit. These 2 mate- rials have a similar microstructure in the sense that they do not contain a hard particulate phase. The microfilled "composi te" which contains approx- imately 37% by weight of a hard par- ticulate phase (particle sizes less the 0.1 p,m), exhibited no significant change in wear rate be tween 120 and 400 grit at both contact stresses. This material showed a min imum wear rate at 800 grit and this behavior was similar to that found for the unfilled r e s i n .

It might have been expected that all 4 materials would show a trend indicating that the larger the particle size of SiC the greater the wear rate at a constant stress. However , only the convent ional composi te illustrated this at both stress levels. There is a pro- found increase in the wear rate of the conventional composi te in the region of 400 grit SiC. In the case of the amalgam, unfilled resin and microfilled "composi te" the change in wear rate with SiC grit numbers was not as marked. A t the lower stress the ratio of the highest to the lowest wear rate for amalgam was 1.8:1, for unfilled resin 1.8:1 and for the microfilled "compo- site" was 1.9:1, whereas the ratio for

2 D e n t a l M a t e r i a l s 1 :1 , 1985

18 Harrison and Moores

2 2 0 S iC Sl ISO 400 S~C

Sl ISO 197 600 SiC

sI 150 197 8OO SiC

sI 150 ,I 97 COHPOSITE

sI I&O 197

197

I I I J I I I I I I ,I 0 20 40 60 80 I00

Par t ic le size ( y m )

Fig. 7. Approximate particle-size distribution. The 5, 50 and 97 levels on the bars indicate the percentage of particles smaller than the corresponding number on the size axis. The com- posite figures are for conventional composite.

according to their wear rates and the ranking orders of the 4 materials are given in Table 2. The anomalous be- havior of the conventional composite leads to changeS in the ranking order with grit size. For the coarsest grit used there was no significant difference be- tween the wear rates of the 3 polymer- based materials and clearly the wear mechanism of the conventional compo- site, in this regime, was not influenced by the presence of a hard, particulate filler. At 180 and 220 grits the ranking orders were similar to those reported in clinical trials. For finer grits the con- ventional composite was the most wear resistant of the materials, indicating that the presence of a hard filler had a profound effect in this regime.

the conventional composite was 6.2:1. At the higher stress the ratios were: amalgam 2.8:1, unfilled resin 2.2:1, mi- crofilled "composite" 2.9:1 and the conventional composite 7.5:1.

The microstructure of the con- ventional composite differs greatly from the other 3 materials inasmuch as it contains approximately 79% by weight of a hard, particulate filler the particle sizes of which are of the same order of magnitude as those of the abrading counterface.

The approximate particle size dis- tributions 6f the conventional com- posite and the SiC particles (used in this study) around 400 grit are shown in Fig. 7. As previously stated, the wear rate of the conventional composite in- creases rapidly in this region. It can be seen from Fig. 7 that for SiC grit numbers equal to and smaller than 400 there is a negligible overlap of particle size distribution with that of the filler in the conventional composite. For grit numbers larger than 400 there is an ap- preciable overlap of the distributions. It is postulated that the relative particle sizes of the two faces in contact leads to the variation in wear rate of the con- ventional composite and that the abra- sive particle size may be divided into two regimes: one where the particle sizes of the abrasive and composite are comparable leading to a low rate of wear and one where the particles of the abrasive are much larger than the com- posite filler, leading to a markedly higher rate of wear.

Dental materials are often ranked

R e f e r e n c e s

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