Wettability (Kemampubasahan) [Compatibility Mode]

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WETTABILITY (KEMAMPUBASAHAN) IKATAN PADA KOMPOSIT

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material komposit

Transcript of Wettability (Kemampubasahan) [Compatibility Mode]

Page 1: Wettability (Kemampubasahan) [Compatibility Mode]

WETTABILITY (KEMAMPUBASAHAN)IKATAN PADA KOMPOSIT

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Drops of water on a hydrophobic surface

Wettability

Good or poor wettability?

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Definisi wettability/kemampubasahan

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Wettability

cos θ = (SV – SL)/ LV

• If θ = 180o, the drop is spherical, no wetting takes place

• θ = 0, perfect wetting• 0o<θ<180o, the degree of wetting increases as θ

decreases.• Often it is considered that the liquid does not wet

the fiber if θ>90o

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• These three quantities determine whether the liquid spreads over the solid, or not; whether it "wets" it.

• This is judged by the contact angle, .

Drops of water on a textile surface before and after addition of wetting agent

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Interface/antarmuka

• Antarmuka = daerah planar dengan ketebalan yang hanya beberapa atom, dimana pada daerah ini terjadi perubahan sifat dari matriks ke penguat.

• Terdapat ketidak-kontinyuan sifat kimia, struktur kristal dan molekular, sifat mekanis dan sifat lainnya.

• Terdapat pada komposit laminat hibrid (ada dua atau lebih partikel penguatnya)

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• Once the matrix has wet the reinforcement, bonding will occur

• For a given system, more than one bonding mechanism may exist at the same time

• The bondings may change during various production stages or during services

Interfacial bonding

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Types of interfacial bonding at interface

1) Mechanical bonding

2) Electrostatic bonding

3) Chemical bonding

4) Reaction or interdiffusion bonding

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Mechanical bonding

-Mechanical interlocking or keying of two interfaces can leads to reasonable bond-The rougher the interface, the interlocking is Greater, hence the mechanical bonding is effective

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• Mechanical bonding is effective when the force is applied parallel to the interface

• If the interface is being pulled apart by tensile forces, the strength is likely to be low unless there is a high density of features (designated A)

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Electrostatic Bonding

-Occur when one surface is positively charged and the other is negatively charge (refer to the above figure)-Interactions are short range and only effective over small distances of the order of atomic dimensions-Surface contamination and entrapped gases will decrease the effectiveness of this bonding

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Chemical bonding

• The bond formed between chemical groups on the reinforcement surfaces (marked X) and compatible groups(marked R) in the matrix

• Strength of chemical bonding depends on the number of bonds per unit area and the type of bond

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Reaction or interdiffusion bonding

-The atoms or molecules of the two components may interdiffuse at the interface- For interfaces involving polymer, this type of bonding can be considered as due to the intertwining of molecules

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• For system involving metals & ceramics, the interdiffusion of species from the two components can produce an interfacial layer of different composition and structure from either of the component

• The interfacial layers also will have different mechanical properties from either matrix or reinforcement

• In MMC, the interfacial layer is often a brittle intermetallic compound

• One of the main reason why interfacial layers are formed is in ceramic and metal matrices is due to the processing at high temperature- diffusion is rapid at high temp; according to the Arrhenius equation)

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• Iakatan antar muka Sangat penting untuk MMC peningkatan kekuatan antarmuka dilakukan dengan metode ELECTROLESS PLATING

• Electroless plating dilakukan dengan mendeposisikan logam pada sebuah substrat dengan media larutan polar sebagai agen produksinya

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• Electroless plating dilakukan dengan mendeposisikan logampada sebuah substrat dengan media larutan polar sebagaiagen produksinya

• Keuntungan electroless plating :

- lebih murah

- menggunakan temperatur rendah dalam prosespelapisannya sehingga mengurangi terjadinya oksidasi padasubstrat

- proses pelapisannya tidak tergantung pada bentuk geometrispesimen substrat

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Teori perekatan (adhesion)

Perekatan mekanik.

• perekat harus penetrasi rongga-rongga pada permukaan dan menggantikan udara yang terjebak pada ikatan antarmuka

• Perekat seringkali mengikat lebih baik terhadap permukaan yang terkelupas daripada terhadap permukaan aslinya.

• Efek yang menguntungkan ini mungkin karena interlocking mekanik, formasi permukaan yang bersih, formasi permukaan yang lebih reaktif dan formasi luas permukaan yang lebih besar

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Perekatan serap (adsorption).

perekatan terjadi akibat dari kontak molekular antara dua material dan gaya-gaya permukaan yang berkembang.

Proses penetapan kontak yang baik antara perekat dan bahan yang direkati (ditempeli) tersebut diketahui sebagai wetting (pembasahan).

untuk membasahi permukaan padat, perekat sebaiknya mempunyai tegangan permukaan yang lebih rendah dari pada tegangan permukaan kritis bahan padat.

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• Teori electrostatic = gaya-gaya electrostatic dalam bentuk lapisan ganda elektrik terbentuk pada ikatan antarmuka perekat dan bahan yang direkati. Gaya-gaya ini bertanggung jawab melawan pemisahan.

• teori diffusi = perekatan timbul melalui inter diffusi molekul di dalam perekat dan bahan yang direkatkan. diterapkan ketika perekat dan bahan yang direkati keduanya adalah polymeric, yang mempunyai kemampuan gerakan molekul rantai panjang. Ikatan yang terbentuk oleh larutan atau thermoplastic pengelasan panas akibat dari diffusi molekul-molekul

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Teori lapisan batas lemah dapat berasal dari perekat, bahan yang direkat, lingkungan, atau kombinasi dari ketiganya.

lapisan batas yang lemah dapat terjadi pada perekat dan bahan yang direkat jika bahan pengotor mendekati permukaan ikatan dan membentuk ikatan yang lemah terhadap substrat.

Ketika kegagalan ikatan terjadi, ini adalah lapisan batas lemah yang gagal, meskipun kegagalan kelihatan terjadi pada ikatan antarmuka perekat dan bahan yang direkat.

Kagagalan sambungan perekat dengan bahan yang direkat akan terjadi secara terpadu di dalam oksida yang lemah. Ketika perekat tidak membasahi substrat, lapisan batas yang lemah dari udara terjebak pada ikatan antarmuka, yang menyebabkan penurunan kekuatan sambungan

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Kompaksi/penekanan

• Peningkatan kompaksi akan memberikan hasil packing yang lebih baik dan menurunkan porositas seluruh partikel mengalami work (strain) hardening

• Tekanan kompaksi dinaikkan jumlah partikel yang mengalami deformasi plastis meningkat, aliran plastis yang homogen terjadi seluruhnya

• Pada tekanan rendah, aliran plastis dipusatkan pada kontak partikel

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Methods for measuring bond strength

• Single fiber test

• Fiber pull-out test (a)

• Melibatkan menarik serat tunggal sebagian tertanam dari blok bahan matriks

• Sulit untuk dilakukan terutama untuk serat rapuh tipis

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Fiber pull-out test (a)

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• Compression test for interfacial shear strength (b)

• The interfacial shear strength (ζ1) may be evaluated using a specimen consisting of a block of matrix materials with a single, embedded short fiber with accurately aligned longitudinal in a center of the specimen (b)

• On testing in compression, shear stresses are set up at the ends of the fibers as a consequence of the difference in elastic properties of the fiber and matrix

• The shear stress eventually leads to debonding at the fiber ends and ζ1 may be evaluated based on;

• ζ1 ~ 2.5 σc (σc is the compressive stress at which debonding occurs- difficult to be determined)

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• Compression test for interfacial tensile strength (c)

• Debonding induced by tensile stresses if a curves, neck specimen with a continuous fiber is tested in compression (c)

• At a compressive stress of σc , the tensile strength σ1of the interface is reached and tensile debonding occurs, σ1 = C σc , C is a constant which depends on Poisson’s ratio and Young’s Modulus of fiber & matrix

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Bulk specimen tests

The simplest method and most widely employed

The tensile strength and shear strength obtained from the 3-point bending test are found to depend on the volume of fibers-not a true values for the bond strength

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• At a given load P, the max. stress σ is given as;

σ = 3PS/2D2B………(1)P= Load, S=span length, D= thickness

B=width

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Micro-indentation test• Employs a standard micro-indentation hardness

tester

• The indentor is loaded with a force, P on to a center of a fiber, whose axis is normal to the surface, and caused the fiber to slide along the matrix-fiber interface

• Suitable for CMC

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Composite Properties

Heat Capacity and density• Can be predicted using Rule of Mixture.

• Density, ρc = ρmVm + ρfVf

• Heat Capacity,Cc =(CmρmVm + Cf ρfVf )/ ρc

• V= volume fraction, m=matrix, c=composite, f= fiber, C= heat capacity

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• Modulus of Elasticity• 2 Models can be used to predict the elastic

modulus of the composites

• (1) Isostrain condition

- Load is applied parallel to the fiber alignment, assume equal deformation in the components

(2) Isostress condition

- Load is applied perpendicular to the fiber alignment

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Tensile elastic modulus vs. volumefraction of fiber.

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• Strength

• Difficult to predict the strength by using the rule of mixture, this is due to the sensitivity of strength toward the matrix and fiber structure

- For example, matrix and fiber structure will be changed during the fabrication process

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• Toughness

• Depends on few factors:

1) Composition and microstructure of the matrix

2) Type, size and orientaion of fiber

3) Processing of composite; effect the microstructure, i.e. voids, distribution of fiber, etc.