GLOSSARY

… “DIY” glossary of materials to have materials science always at your fingertips

Simultaneous thermal analysis (STA):

is a technique that allows to relate the thermal events (Differential Thermal Analysis – DTA) which affect a sample with its weight variation (Thermogravimetry – TG). In this way, it is possible to discern between endothermic (heat-absorbing) events, exothermic (heat-releasing) events and those that determine a mass loss (i.e. oxidation, degradation, combustion).

Differential scanning calorimetry (DSC):

is a thermo-analytic technique used to evaluate phase transitions by measuring heat exchange between sample and reference material as a function of temperature and time.

Microstructural characteristics:

are the small-scale structural features, such as grain size and shape, phase distribution and contaminants, generally visible by means of optical microscopy. Microstructural study is of paramount importance for industry, since it strongly influences physical properties of material, such as strength, toughness, hardness etc.

Advanced ceramics:

are crystalline inorganic solids, whose atoms are ionic or covalent bonded. They are generally fragile but, they possess considerable hardness, excellent resistance to high temperature (even above 1400°C) and low thermal and electric conductivity. They are used as insulators, anti-wear protective coatings and piezoelectrics.

Ant nest corrosion:

is one of the several corrosion typologies that affects copper and its alloys and it is hard to detect and prevent. The corrosion pattern is characterized by microscopic interconnecting tunnels similar to ant nest, which propagate rapidly inside material. Depending on component’s thickness pinhole leaking can happen in very short time (weeks or months). During the corrosion process, cuprous oxide (Cu₂O) is deposited on the inner pipe walls. This process is triggered by the concurrent presence of water, oxygen and carboxylic acids.

Crazing:

is a typical damage that occurs in amorphous polymers consisting in narrow zones of very deformed material characterized by microvoids and highly oriented fibrils. These areas present reduced density and they are particularly critical since they can become real cracks if further mechanical load is applied. Since crazes scatter light, they are readily visible as whitened planes perpendicular to stress direction. Their formation is favoured by the presence of organic liquids as well as temperature increase.

Cristallinity:

some polymeric materials possess long chain segments regularly oriented which show physical properties similar to crystalline solids. The ability of a polymer to crystallize is influenced by chemical structure and ramifications: crystallinity is favoured for simple and linear molecules, whereas complex structures and side branches hinder it. The degree of crystallinity, which indicates the fraction of crystalline phase in a polymer, depends also upon cooling rate: molecules can assume an ordered configuration if they have enough time to align.

Environmental degradation:

the environment can be aggressive towards a certain component, leading to various types of defects. Light can trigger photo-oxidation reactions, sudden temperature changes can induce stress, cracking and damage. Water, air and various reagents (i.e. those often found in foods or cleaning products) can trigger several chemical reactions that degrade the material.

Dimples:

are microvoids which develop within a metal, next to local defects or at the interface between different phases, as a result of mechanical load. Their coalescence leads to a ductile fracture.

Elastomer:

is a polymeric material capable to withstand a considerable elastic deformation and then recover its original size and shape. Elastomers can be divided into thermoplastic and thermosetting; the latter are obtained by a process generally known as “vulcanization”.

Failure analysis:

is typically performed when a system, equipment or component is subjected to an early, sudden or unexpected failure. The aim of this investigation is to determine the causes that led to breakage and prevent the repetition of such event. Depending on the nature of the failed part, different techniques can be used, such as optical or electron microscopy, spectroscopy and X-ray diffraction.

Photo-oxidation:

is a degradation process typical for polymers activated by the absorption of the ultraviolet component of light. It causes chemical bonds breakage, free radical formation and generation of oxidized chemical groups. The typical macroscopic consequences that can be found in plastics are colour change and reduction of mechanical properties.

Degree of cross-linking:

cross-linking (or cure) indicates the process of bonding polymer chains to build up a three-dimensional structure. The degree of cross-linking describes the conversion achieved by a thermosetting polymer during curing: it varies from 0 (reaction beginning) to 1 (reaction completed). It can be measured by thermal methods or by spectroscopy.

Artificial wheathering:

is performed to simulate the effects of the external environment, in particular solar radiation and rain. Samples are exposed to artificial light sources and water spraying, alternating dark and light cycles. This type of test is known as “accelerated” because the obtained results are comparable to those of samples exposed to natural atmospheric conditions for longer time.

Composite Materials:

are composed of two or more distinct phases – the former defined as matrix, the latter as reinforcement – which differ from each other at a macroscopic level. Usually the components can be physically identified and an interface is present between them. Composites can be distinguished according to the nature of matrix (polymeric, ceramic or metallic) and the morphology of the reinforcements (particles, short and long fibres, fabrics, etc.).

Non-conformity:

is the deviation of one or more characteristics related to a product, system, procedure or process from a standard, a specification or a requirement.

Enviromental sustainability:

is a development model that plans a responsible interaction with the environment to avoid depletion of natural resources, since they are not infinite. In this way, the satisfaction of the present generation needs does not compromise the development possibilities of future ones.

Fourier transform infrared (FTIR) spectroscopy:

is a technique that allows to investigate the chemical nature of materials by analysing the vibrational bands associated to different functional groups. The identification of chemical species is usually carried out by comparing samples spectra with those collected on reference materials.

Raman Spectroscopy:

is a technique based on the interaction between a monochromatic electromagnetic radiation and sample. A small fraction of the incident photons is inelastically scattered by the sample (Raman effect), with an energy higher or lower than incident light. This energy difference corresponds to vibrational energies of the molecules; it is therefore possible to gather information on sample composition and molecular structure.

Glass transition temperature:

is defined as the temperature at which a polymer changes its mechanical behaviour from rigid and brittle to soft and deformable. Amorphous and semi-crystalline polymers are characterized by this behaviour.

Stress corrosion:

is a failure mode typical of metals caused by the combination of mechanical load and corrosive environment. It can cause a sudden and unexpected breakage of usually ductile materials.

Thermosetting:

is a polymer material that after curing process realized by means of heating or chemical reaction, cannot be melted by a simple temperature increase.

Thermoplastic:

is a polymeric material that can be repeatedly softened by heating and hardened by cooling. In its softened state, it can be shaped into components by moulding or extrusion.