Davim J. Paulo (editor). Machining. Fundamentals and Recent Advances

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160 A. Pramanik et al.

right tool for machining any material to minimize the machining cost is a critical

issue. It is common practice to perform experiments and select cutting tools and

conditions for a particular application based mainly on tool wear and surface fin-

ish. For relatively new MMC materials, the above procedure was adopted for

selecting suitable tools and machining conditions. Since hard ceramic reinforce-

ments in the MMC have hardness comparable to some cutting tool materials, it is

natural that MMCs are difficult to machine.

Comparatively high tool wear, due to tool–particle interactions, is affected by

cutting parameters and the size and volume percentage of reinforcements. At

high speed and low feed, tool–particle interactions become more frequent and

substantial, which accelerates tool wear [4,

48,

49]. Tool wear is lower for small

size and low volume percentage of reinforcement and high-hardness cutting tools

[67,

68].

Researchers have tested almost all available tool materials to machine different

types of MMCs. PCD shows comparatively better performance over any other tool

material. However, because of the high cost of PCD tools, use of PCBN [69], CVD

diamond-coated [69], TiCN/TiN-coated carbide [5], triple-layer TiC/Al2O3/TiCN-

coated carbide [71] and uncoated carbide [5,

23,

72,

73] tools for machining of

MMCs has been investigated.

It has been argued that the dominant wear mechanisms during machining of MMC

are two- and three-body abrasions and that these are due to the presence of hard

reinforced particles and debonded tool material grains [3,

48,

67,

74,

76].

These cause tool pitting, chipping, microcracking and fatigue [75,

78]. Some re-

searchers [74,

77] have argued that adhesion is also a cause of tool wear in addi-

tion to abrasion, as thin films of the workpiece material were found to be adhered

to the worn areas. They considered that the abrasion was associated with micro-

mechanical damage rather than microcutting. Chemical wear during machining of

MMC has not been reported for any tool material. This is because the constituents

of MMC, i.e., matrix material and reinforced particles (e.g., SiC and Al

) are

chemically inert to almost all cutting tools (e.g., PCD, CVD-coated, carbide tools)

under almost any conditions.

For coated/uncoated tungsten carbide tools, Arsecularatne et al. [79] pointed

out that, when machining MMC work materials, tool wear is due to mechanical

abrasion.

The increased thickness of diamond CVD coating increase the performance of

the tool and the thick (500 μm) CVD diamond coating may be considered a com-

petitor to PCD [77,

78]. Bonding between the substrate and the coating is critical

to this tool performance. Coating failure is the dominant wear mechanism for

diamond CVD-coated tools during machining of MMCs [70]. Coating failure

accelerates at high temperature due to different thermal expansions between the

coating and the substrate.

Most of the published research has investigated the performance of PCD tools

during machining of MMCs. Wear mechanisms such as abrasion, adhesion and

microcracking and fatigue have been used to explain tool wear [79]. This indicates

that tool wear mechanism when machining of MMCs is not yet fully understood.

Machining of Particulate-Reinforced Metal Matrix Composites 161

5.4.2 Modelling of Tool Wear

A number of attempts to model tool wear and correlate it with the machining pa-

rameters can be found. Most researchers developed a tool wear model assuming

MMC to be a monolithic metal, i.e., the influence of the particles was not consid-

ered explicitly in the model. They either extended Taylor’s equation [80–82]

based on experimential analysis or developed empirical relations [20,

83] of tool

wear with machining parameters from experimental data. To date only four mod-

els are available where effects of particles are considered on tool wear. These

assume abrasion to be the dominant wear mechanism and are described below.

Li et al. [68] developed a model to calculate the critical weight percentage of

reinforcement for tool wear acceleration after a comprehensive study of the tool

wear mechanism in cutting MMCs. Similar to Davim et al. [7] they argued that

wear was due to two- and three-body abrasions, and that wear was accelerated by

interference between the reinforcement particles that was associated with a critical

weight percentage. Based on these observations, an analytical model was devel-

oped to predict the critical reinforcement weight ratio as a function of the densities

of the reinforcement and the matrix and of the radius of the reinforcement particles

and the cutting tool edge. They assumed that particles were spherical and that the

maximum particle displacement was the radius (or function of radius) of the im-

pacted reinforced particle, which occurs when the impact point is near the parti-

cle’s centroid. A comparision between predictions and their experimental results

showed reasonable agreement. The main disadvantage of this model is that it

could not correlate tool wear with cutting conditions and tool material properties.

Kishawy et al. [84] also developed a model to predict flank wear rate consider-

ing two- and three-body abrasion between the MMC and tool, which were formu-

lated using modified Archard’s and Rabinowicz’s relations, respectively. The rate

of volume loss of the cutting tool was calculated by incorporating the contribu-

tions of the two types of abrasions by taking the root-mean-square value. The rate

of volume loss was correlated with length of flank wear land by considering the

geometry of the cutting tool. A comparision between predictions and their experi-

mental results showed reasonable agreement.

Kannan et al. [85] proposed a tool wear model to predict flank wear based on

Holm–Archard theory [86] by correlating hardness and energy consumption. The

energy consumed for machining was determined in terms of the power required by

the cutting system, tool volume loss, tool hardness and the properties of the ce-

ramic particulate reinforcement. The power required by the cutting system was

calculated as the product of the energy per unit volume of metal removed and the

material removal rate. From these expressions the tool volume loss was calculated,

which was correlated with the flank wear land width using a two-dimensional

geometry of cutting tool.

Pedersen et al. [87] proposed an empirical wear model to predict flank wear

based on the probability of four possible tool–particle interactions depending on

the tool–particle orientations. Based on the probability of tool–particle interaction,

it was assumed that only two orientations of these four take part in tool wear sig-

nificantly. They accounted for workpiece and cutting tool properties in addition to

the traditionally used cutting conditions. This model is an expansion of the trad-

162 A. Pramanik et al.

itionally extended Taylor’s tool life equation based on Rabinowicz’s wear equa-

tion [86]. A major disadvantage of this model is the large number of empirical

constants/exponents which have to be determined experimentally, necessitating

time-consuming and costly machining tests.

As discussed in this section, almost all available tool materials (e.g.,

coated/uncoated tungsten carbide, PCBN and PCD) have been used to machine

MMCs. Available experimental results show that the presence of reinforcement

particles significantly influences tool wear. Wear of coated/uncoated tungsten

carbide tools has been attributed to mechanical abrasion. On the other hand, wear

mechanisms such as abrasion, adhesion, and microcracking and fatigue have been

used to explain wear of PCD tools. This indicates that wear mechanism(s) of PCD

tools are not yet fully understood. Some models are also available in the literature

for predicting tool wear. Comparisons between model predictions and investiga-

tors’ experimental results seem to show good agreement. However, a comprehen-

sive comparison of predicted and experimental results including those available in

the literature has yet to be carried out.

Acknowledgements

The authors wish to thank the Australian Research Council for financial assis-

tance. A. P. has been supported by IPRS and IPA.

References

[1] Rashad RM, El-Hossainy TM (2006) Machinability of 7116 structural aluminum

alloy. Mater Manuf Process 21: 23–27

[2] Durante S, Rutelli G, Rabezzana F (1997) Aluminum-based MMC machining with

diamond-coated cutting tools. Surf Coatings Technol 94–95: 632–640

[3] Heath PJ (2001) Developments in the applications of PCD tooling. J Mater Process

Technol 116: 31–38

[4] Davim JP (2002) Diamond tool performance in machining metal–matrix composites.

J Mater Process Technol 128: 100–105

[5] Pedersen W, Ramulu M (2006) Facing SiCp/Mg metal matrix composites with car-

bide tools. J Mater Process Technol 172: 417–423

[6] Chambers AR (1996) The machinability of light alloy MMCs. Composites: Part A

27: 143–147

[7] Davim JP, Baptista AM (2000) Relationship between cutting force and PCD cutting

tool wear in machining silicon carbide reinforced aluminium. J Mater Process Tech-

nol 103: 417–423

[8] Hung NP, Loh NL, Venkatesh VC (1999) Machining of metal matrix composites, in

Machining of Ceramics and Composites: ed by S. Jahanmir, M. Ramulu and P. Ko-

shy, 1999, Marcel Dekker, New York. Basel, ISBN 082470178X

[9] Wang Z, Chen T, Lloyd DJ (1993) Stress distribution in particulate-reinforced metal-

matrix composites subjected to external load. Metall Trans 24(A): 197–207

[10] Huda D, El-Baradie MA, Hashmi MSJ (1994) Analytical study for the stress analysis

of metal matix composites. J Mater Process Technol 45: 429–434

Machining of Particulate-Reinforced Metal Matrix Composites 163

[11] Shi N, Arsenault RJ (1991) Analytical evaluation of the thermal residual stresses in

Si/Al composites. JSME Int J 34(2): 143–155

[12] Zhao B, Liu CS, Zhu XS, Xu KW (2002) Research on the vibration cutting perform-

ance of particle reinforced metallic matrix composites SiCp/Al. J Mater Process

Technol 129(1–3): 380–384

[13] Brun MK, Lee M, Gorsler F (1985) Wear characteristics of various hard materials for

machining SiC-reinforced aluminum alloy. Wear 104(1): 21–29

[14] Narutaki N (1996) Machining of MMCs. VDI Berichte NR 1276: 359–370

[15] Clyne TW, Withers PJ (1993) An introduction to metal matrix composites, 1st edi-

tion. Cambridge University Press, ISBN 0-521-41808-9

[16] Pramanik A, Zhang LC, Arsecularatne JA (2007) An FEM investigation into the

behaviour of metal matrix composites: tool-particle interaction during orthogonal cut-

ting. Int J Mach Tools Manuf 47: 1497–1506

[17] Mussert KM, Vellinga WP, Bakker A, Zwaag SVD (2002) A nano-indentation study

on the mechanical behaviour of the matrix material in an AA6061 – Al

MMC. J

Mater Sci 37(4): 789–794

[18] Leggoe JW (2004) Determination of the elastic modulus of microscale ceramic parti-

cles via nanoindentation. J Mater Res 19(8): 2437–2447

[19] Pramanik A, Zhang LC, Arsecularatne JA (2007) Micro-indentation of metal matrix

composites – an FEM analysis. Key Eng Mater 340–341: 563–570

[20] Lin JT, Bhattacharyya D, Lane C (1995) Machinability of a silicon carbide reinforced

aluminium metal matrix composite. Wear 181–183: 883–888

[21] Pramanik A, Zhang LC, Arsecularatne J. Machining of metal matrix composites:

effect of ceramic particles on residual stress, surface roughness and chip formation.

Int J Mach Tools Manuf (under review)

[22] Lin JT (1997) Bhattacharyya, D. and Ferguson, W.G., Chip formation in the machin-

ing of SiC particle reinforced aluminium matrix composites. Compos Sci Technol 58:

285–291

[23] Karthikeyan R, Ganesan G, Nagarazan RS, Pai BC (2001) A critical study on ma-

chining of Al/SiC composites. Mater Manuf Process 16(1): 47–60

[24] Joshi SS, Ramakrishnan N, Ramakrishnan P (2001) Micro-structural analysis of chip

formation during orthogonal machining of Al/SiCp composites. J Eng Mater Technol

123: 315–321

[25] Hung NP, Yeo SH, Lee KK, Ng KJ (1998) Chip formation in machining particle-

reinforced metal matrix composites. Mater Manuf Process 13(1): 85–100

[26] Ozcatalbas Y (2003a) Investigation of the machinability behaviour of Al

rein-

forced Al-based composite produced by mechanical alloying technique. Compos Sci

Technol 63(1): 53–61

[27] Ozcatalbas Y (2003b) Chip and built-up edge formation in the machining of in situ

–Al composite. Mater Des 24(3): 215–221

[28] Quan, YM, Zhou, ZH and Ye, BY (1999), Cutting process and chip appearance of

aluminium matrix composites reinforced by SiC particle, J Mater Process Technol,

91(1), 231–235

[29] Pramanik A, Zhang LC, Arsecularatne JA (2006) Prediction of cutting forces in

machining of Metal Matrix Composites. Int J Mach Tools Manuf 46: 1795–1803

[30] Dieter GE (1988) Mechanical Metallurgy, SI Metric Edition, McGraw-Hill, UK

[31] Clausen AH, Borvik T, Hopperstad OS, Benallal A (2004) Flow and fracture charac-

teristics of aluminium alloy AA5083–H116 as function of strain rate, temperature

and triaxiality. Mater Sci Eng A364: 260–272

[32] Wulf GL (1978) The high strain rate compression of 7039 aluminium. Int J Mech

Sci 20(9): 609–615

164 A. Pramanik et al.

[33] Smerd R, Winkler S, Salisbury C, Worswick M, Lloyd D, Finn M (2005) High strain

rate tensile testing of automotive aluminium alloy sheet. Int J Impact Eng 32:

541−560

[34] Davim JP (2007) Application of Merchant theory in machining particulate metal

matrix composites. Mater Des 10: 2684–2687

[35] Oxley PLB (1989) The mechanics of machining: an analytical approach to assessing

machinability. Ellis Horwood, Chichester

[36] Jaspers SPFC, Dautzenberg JH (2002) Material behaviour in metal cutting: strains,

strain rates and temperatures in chip formation. J Mater Process Technol 121:

123−135

[37] Li Y, Ramesha KT, Chin ESC (2004) The mechanical response of an A359/SiCp

MMC and the A359 aluminium matrix to dynamic shearing deformations. Mater Sci

Eng A 382: 162–170

[38] Li Y, Ramesh KT (1998) Influence of particle volume fraction, shape, and aspect

ratio on the behaviour of particle-reinforced metal matrix composites at high rates of

strain. Acta Mater 46(16): 5633–5646

[39] Li Y, Ramesh KT, Chin ESC (2000) The compressive viscoplastic response of an

A359/SiCp metal-matrix composite and of the A359 aluminium alloy matrix. Int J

Solids Struct 37:7547–7562

[40] Zhang ZF, Zhang LC, Mai YW (1995) Particle effects on friction and wear of alu-

minium matrix composites. J Mater Sci 30(23): 5999–6004

[41] Chichili DR, Ramesh KT (1995) Dynamic failure mechanisms in a 6061-T6

Al/Al

metal-matrix composite. Int J Solids Struct 32(17–18): 2609–2626

[42] Yadav S, Chichili DR, Ramesh KT (1995) Mechanical response of a 6061-T6

Al/Al

metal matrix composite at high rates of deformation. Acta Metall Materialia

43(12):4453

[43] Oxley PLB (1962) Shear angle solutions in orthogonal machining. Int J Mach Tool

Des Res 2(3): 219–229

[44] Ng E, Aspinwall DK (2002) The effect of workpiece hardness and cutting speed on

the machinability of AISI H13 hot work die steel when using PCBN tooling. Trans

ASME 124:588–594

[45] Kishawy HA, Kannan S, Balazinski M (2004) An energy based analytical force

model for orthogonal cutting of metal matrix composites. Ann CIRP 53(1): 91–94

[46] El-Gallab M, Sklad M (1998) Machining of Al/SiC particulate metal-matrix compos-

ites Part II: Workpiece surface integrity. J Mater Process Technol 83: 277−285

[47] Hong SY, Ding Y, Ekkens RG (1999) Improving low carbon steel chip breakability

by cryogenic chip cooling. Int J Mach Tools Manuf 39: 1065–1085

[48] Ding X, Liew WYH, Liu XD (2005) Evaluation of machining performance of MMC

with PCBN and PCD tools. Wear 259: 1225–1234

[49] El-Gallab M, Sklad M (1998b) Machining of Al/SiC particulate metal matrix com-

posites. Part II: work surface integrity. J Mater Process Technol 83: 277–285

[50] Cheung CF, Chan KC, To S, Lee WB (2002) Effect of reinforcement in ultra-

precision machining of Al6061/SiC metal matrix composites. Scripta Mater 4: 77–82

[51] Sahin Y, Kok M, Celik H (2002) Tool wear and surface roughness of Al

particle-

reinforced aluminium alloy composites. J Mater Process Technol 128: 280−291

[52] Monaghan J (1998b) Factors affecting the machinability of Al/SiC metal matrix

composites. Key Eng Mater 138–140: 545–574

[53] Lin CB, Hung YW, Liu W-C, Kang S-W (2001) Machining and Fuidity of

356Al/SiC(p) composites. J Mater Process Technol 110: 152–159

[54] Capello E (2005) Residual stress in turning Part I: Influence of process parameters. J

Mater Process Technol 160: 221–228

Machining of Particulate-Reinforced Metal Matrix Composites 165

[55] El-Axir MH (2002) A method of modeling residual stress distribution in turning for

different materials. Int J Mach Tools Manuf 42: 1055–1063

[56] Brinksmeier E, Cammett JT, Koenig W, Leskovar P, Peters J, Toenshoff HK (1982)

Residual stresses – measurement and causes in machining processes. Ann CIRP

31(2): 491–510

[57] Kannan S, Kishawy HA (2006) Surface characteristics of machined aluminium metal

matrix composites. Int J Mach Tools Manuf 46: 2017–2025

[58] Davim JP (2001) Turning particulate metal matrix composites: experimental study of

the evolution of the cutting forces, tool wear and workpiece surface roughness with

the cutting time. J Eng Manuf Proc Inst Mech Eng 215 B: 371–376

[59] Davim JP, Silva J, Baptista AM (2007b) Experimental cutting model of metal matrix

composites (MMCs). J Mater Process Technol 183(2–3): 358–362

[60] Merchant ME (1944) Mechanics of the metal cutting process. I. Orthogonal cutting

and type 2 chip. J Appl Phys 16: 267–275

[61] Lee EH, Shaffer BW (1951) The theory of plasticity applied to a problem of machin-

ing.J Appl Mech December: 15–20

[62] Kobayashi S, Thomsen EG (1959) Some observations on shearing process in metal

cutting. Transactions ASME. J Eng Ind August: 251–261

[63] Pugh HLD (1958) Mechanics of cutting process, Proceedings of Conference on

Technology of Engineering Manufacture, The Institute of Mechanical Engineers, p.

237–254

[64] Waldorf DJ (2004) A simplified model for ploughing forces in turning. Trans

NAMRI SME 32: 447–454

[65] Yan C, Zhang LC (1995) Single-point scratching of 6061 Al Alloy reinforced by

different ceramic particles. Appl Compos Mater 1:431–447

[66] Muller HK, Nau B (1998) Fluid Sealing Technology Marcel Dekker, Chapter 14,

p. 294

[67] Yanming Q, Zehua Z (2000) Tool wear and its mechanism for cutting SiC particle-

reinforced aluminium matrix composites. J Mater Process Technol 100(1): 194−199

[68] Li X, Seah WKH (2001) Tool wear acceleration in relation to workpiece reinforce-

ment percentage in cutting of metal matrix composites. Wear 247(2): 161−171

[69] Sreejith PS (2006) Tool wear of binderless PCBN tool during machining of particu-

late reinforced MMC. Tribol Lett 22 (3): 265–269

[70] Chou YK, Liu J (2005) CVD diamond tool performance in metal matrix composite

machining. Surf Coatings Technol 200: 1872–1878

[71] Ciftci I, Turker M, Ulvi S (2004) Evaluation of tool wear when machining

SiC

-reinforced Al-2014 alloy matrix composites. Mater Des 25: 251–255

[72] Varadarajana YS, Vijayaraghavan L, Krishnamurthy R (2006) Performance en-

hancement through microwave irradiation of K20 carbide tool machining Al/SiC

metal matrix composite. J Mater Process Technol 173(2):185–193

[73] Kilickap E, Cakir O, Aksoy M, Inan A (2005) Study of tool wear and surface rough-

ness in machining of homogenised SiC-p reinforced aluminium metal matrix com-

posite. J Mater Process Technol 164–165: 862–867

[74] Hooper RM, Henshall JL, Klopfer A (1999) The wear of polycrystalline diamond

tools used in the cutting of metal matrix composites. Int J Refractory Metals Hard

Mater 17: 103–109

[75] El-Gallab M, Sklad M (2000) Machining of Al/SiC particulate metal matrix compos-

ites part III: comprehensive tool wear models comprehensive tool wear models. J

Mater Process Technol 101:10–20

[76] Weinert K (1993) A consideration of tool wear mechanism when machining metal

matrix composite (MMC). Ann CIRP 42: 95–98

166 A. Pramanik et al.

[77] Andrewes CJE, Feng H-Y, Lau WM (2000) Machining of an aluminum/SiC compos-

ite using diamond inserts. J Mater Process Technol 102: 25–29

[78] D’Errico GE, Calzavarini R (2001) Turning of metal matrix composites. J Mater

Process Technol 119: 257–260

[79] Arsecularatne JA, Zhang LC, Montross C (2006) Wear and tool life of tungsten

carbide, PCBN and PCD cutting tools. Int J Mach Tools Manuf 46: 482–491

[80] Masounave J, Litwin J, Hamelin D (1994) Prediction of tool life in turning alumin-

ium matrix composites. Mater Des15(5): 287–293

[81] Hung NP, Boey FYC, Khor KA, Phua YS, Lee HF (1996a) Machinability of alumi-

num alloys reinforced with silicon carbide particulates. J Mater Process Technol 56:

966–977

[82] Hung NP, Zhong CH (1996b) Cumulative tool wear in machining metal matrix com-

posites Part I: Modelling. J Mater Process Technol 58: 109–l 13

[83] Davim JP (2003) Design of optimization of cutting parameters for turning metal

matrix composites based on the orthogonal arrays. J Mater Process Technol

132(1−3): 340−344

[84] Kishawy HA, Kannan S, Balazinski M (2005) Analytical modeling of tool wear

progression during turning particulate reinforced metal matrix composites. Ann

CIRP, 54(1): 55−58

[85] Kannan S, Kishawy HA, Deiab IM, Surappa MK (2005) Modeling of tool flank wear

progression during orthogonal machining of metal matrix composites, Trans North

Am Manuf Res Inst SME NAMRC 33: 605–612

[86] Rabinowicz E (1995) Friction and Wear of Materials. Wiley Interscience, 2

edn,

pp. 169–170

[87] Pedersen WE, Ramulu M (2005) Proposed tool wear model for machining particle

reinforced metal matrix composites. Trans North Am Manuf Res Inst SME Papers

Presented at NAMRC 33: 549–556

Drilling Polymeric Matrix Composites

Edoardo Capello

, Antonio Langella

, Luigi Nele

, Alfonso Paoletti

, Loredana

Santo

, Vincenzo Tagliaferri

Dipartimento di Meccanica, Politecnico di Milano, piazza Leonardo da Vinci 32,

20133 Milano, Italy.

E-mail: edoardo.capello@polimi.it

Department of Materials and Engineering Production, University of Naples Fede-

rico II, Piazzale Tecchio 32, 80125 Napoli, Italy.

E-mail: antgella@unina.it, luigi.nele@unina.it

Università di L’Aquila, Piazza V. Rivera 1, 67100 L’Aquila, Italy.

E-mail: alpao@ing.univaq.it

Dipartimento di Ingegneria Meccanica, Università di Roma “Tor Vergata”,

via del Politecnico 1, 00133 Roma, Italy.

E-mail: loredana.santo@uniroma2.it, tagliaferri@mec.uniroma2.it

This chapter presents the basics of drilling of polymeric matrix composites

(PMCs). PMCs are becoming widely used in the manufacturing of products where

a high mechanical strength must be accompanied by a low weight. However, the

machining of PMCs implies coping with problems that are not encountered when

machining other materials. Drilling is a particularly critical operation for PMCs

laminates because the large concentrated forces generated can lead to widespread

damage. This damage causes aesthetic problems but, more importantly, may com-

promise the mechanical properties of the finished part.

6.1 Introduction

6.1.1 What Are Polymeric Matrix Composites?

Composite materials are made up of two or more constituents [1–8]. The principle

behind the study of composite materials and their applications is based on the

possibility of using materials with specific characteristics which, when joined

together in an appropriate manner, create a single composite material with final

properties that are not found in any of the raw materials.

One of the constituents, called the matrix, usually determines the characteristics

of the composite material primarily according to the field of application, the tem-

perature of use and the weight. Its role is to contain another constituent, called the

168 E. Capello et al.

reinforcement, which determines the principal mechanical characteristics of the

composite material, tensile modulus and mechanical resistance.

In nature, it is possible to find various examples of natural composite materials,

such as bone or wood. An artificial composite material can be obtained using

various types of matrix and with various types of reinforcement. The reinforce-

ment may be in the form of filaments, called fibres (long or short), or in the form

of particles. In theory, all materials can be used for the production of fibres, but in

practice only a limited number of materials can be selected, and even fewer of

these have those mechanical characteristics that are of interest in engineering ap-

plications.

Both matrix and fibres can be metallic, ceramic or polymeric, and the compos-

ite materials, classified according to matrix type, can be divided as follows:

• polymeric composites

• metallic composites

• ceramic composites

Polymeric matrix composite materials (PMCs) have a matrix which is made of

a thermoset or a thermoplastic polymeric resin. Polymeric matrices are made up of

rigid polymeric chains obtained from liquid monomers by means of a process of

chemical transformation known as polymerization.

Thermoplastic matrices are polymers that have polymeric chains without trans-

verse bonds and thus, after polymerization, an increase in temperature leads to

a reduction in viscosity and to a melting of the matrix. The melting from a solid

state develops through a gummy state, in a manner which is reversible.

Thermoset matrices are, on the other hand, characterized by the presence of

strong transverse bonds which are created during polymerization, and which make

the process not reversible. Indeed, an increase in temperature after polymerization

does not initially generate substantial variations in the matrix properties, while

beyond a certain limit it causes the total degradation of the matrix.

With the use of thermoplastic matrices, the component manufacturer uses

a semi-finished product in which the polymerization reactions have already taken

place and are guaranteed by the producer of the matrix. These semi-finished prod-

ucts need only to be modelled to obtain the required shape for the final compo-

nent. As opposed to thermoset matrices, the application of thermoplastic matrices

in composite materials has, in the past, been hindered by a low softening tempera-

ture, a lower tensile modulus and by reduced adhesion between fibres and matrix.

Some types of thermoplastic matrices with properties similar to those of thermoset

matrices are currently available, but with the prospect of lower transformation

costs. The main thermoplastic matrices used in polymeric composite materials are:

poly-ether-ether-ketones (PEEKs), poly-phenylene-sulphide (PPS), and poly-ether-

imide (PEI).

With the use of thermoset matrices, the polymerization process must be carried

out during the manufacturing cycle of the component, and this requires a rigid,

more careful control of the process. The fact that thermoset matrices are so widely

used is due to their strong mechanical properties and to the low cost of the matri-

ces. Moreover, manual and semi-manual processing can be easily performed.

Drilling Polymeric Matrix Composites 169

The most commonly used types of resin are polyester and epoxy. Unsaturated

polyester resins have good mechanical characteristics, are easily workable and

polymerize at room temperature. Unsaturated polyester resins can be classified

according to the constitutional groups that appear in the chain structure. Among

the most common are the ortophtalic polyesters, the isophtalics, the bisphenols and

the dicyclopentadienyls.

Bicomponent or monocomponent epoxy resins have better mechanical proper-

ties compared to polyester resins, although they have a higher cost. Consequently,

their application is generally limited to advanced applications in the aerospace

industry.

The most commonly used types of reinforcement are glass, carbon and ara-

midic fibres.

The most common commercial formats of reinforcements for polymeric matrix

composites are:

• Mat: The fibres are arranged on a plane in a random manner and are

lightly compressed and treated with an adhesive. Alternatively, the unifilo

mat can be used, in which a single thread is arranged on a plane in a casual

manner. The mats are used to create quasi-isotropic composites with mod-

erate mechanical characteristics;

• Roving: Long fibres are wound around bobbins, from which they are un-

wound in bundles and used as continuous fibres or cut to obtain short fi-

bres;

• Woven – fabric: Continuous long fibres are braided together to form a bi-

dimensional or tridimensional material;

• Pre-pregs: These are semi-finished products widely used in advanced ap-

plications, in which fibres have been impregnated with the right amount of

thermoset resin and kept at low temperature, away from the light.

The processes used in the manufacture of polymeric composite materials allow the

fibre and matrix volumes to be selected in such a way that particular requirements

can be met. The quantity of fibres present usually reaches 60% of the available

volume, while the matrix occupies the remaining 40%. The properties of the com-

posite material obviously depend on the properties of the reinforcement and the

matrix (Tables 6.1 and 6.2), and on the quantity of these.

A simple rule to predict in a first approximation a property of the composite is

the so-called mixture rule which states that:

=⋅+⋅

cffmm

PPV PV (6.1)

in which P

is the resulting property of the composite to be determined, P

and P

are the properties of the matrix and the reinforcement and V

and V

are the volu-

metric fractions of the matrix and reinforcement present, respectively.

The reinforcement and the matrix are arranged on a plane, forming a so-called

lamina in which the fibres (short or continuous) are suspended in the matrix (Fig-

ure 6.1). The fibres may be placed casually or they may be aligned. The study of

interaction between fibres and matrix is known as micromechanics.