Effect of Ski Base Material on Structure Quality - Part No. 2

 The Importance of Dimensional Stability in Wet Conditions

Some ski base manufacturers consider dimensional stability to be one of the decisive material properties, particularly for bases intended for warm and wet snow conditions.

In colder conditions, contact occurs predominantly between the ski base and individual ice grains, whose surfaces are covered by a very thin quasi-liquid layer, referred to as the QLL. At the points of actual contact, this layer can act as a highly effective boundary lubricant.

As snow temperature and moisture increase, or as a result of frictional heating, an additional and substantially thicker layer of free water may form in the contact zone. This can be described as a secondary water film.

Under certain conditions, a thin and discontinuous layer of water can reduce friction. Once the secondary water film becomes excessively thick and continuous, however, and is not removed from the contact zone quickly enough, its effect changes. Compared with the very thin QLL on the surface of the ice grains, it may no longer function as an effective lubricant, and other adverse tribological phenomena begin to dominate. These include capillary and suction forces, a substantial increase in the real contact area, viscous resistance during the displacement and shearing of water, non-uniform pressure distribution within the water film, and hydrodynamic losses associated with changes in flow direction and local turbulence.

The ski base structure plays a fundamental role in limiting these adverse effects of the secondary water film. Its purpose is not merely to reduce the contact area mechanically, but also to create a system of grooves and channels that captures, guides and removes water from the contact zone as quickly as possible.

For the structure to perform this function effectively, it must retain the geometry in which it was designed and produced. This remains true when the structure operates in combination with glide waxes or other surface treatments.

If its grooves deform, flatten or partially close under load, their cross-section, depth and water-drainage capacity change. The structure may then cease to function in the way for which it was originally designed.

The requirement for dimensional stability is particularly important in alpine skiing. At high speeds and under substantial dynamic loads, for example in a sharp turn, the ski is subjected to significant loading that is often concentrated on one side.

A soft and elastic ski base may yield locally under this pressure. The geometry of individual grooves can therefore change precisely when they are required to remove the greatest amount of water. A reduction in groove depth or cross-section may restrict water flow and increase the proportion of the secondary water film within the contact zone.

For this reason, some ski bases intended for warm conditions and alpine disciplines have a modulus of elasticity of approximately 950 to 1,050 MPa, equivalent to 0.95 to 1.05 GPa.

A higher E-modulus means that the material undergoes less elastic deformation under the same load. The structure therefore retains its original shape, groove depth and functional cross-section more effectively, even under high local loads.

The E-modulus value cannot, however, be assessed in isolation. It also depends on the measurement temperature, the test method used, the loading rate, specimen orientation, and whether the manufacturer reports a tensile, compressive or flexural modulus. Direct comparison of different ski bases therefore requires values obtained using the same test method and under comparable conditions.

Stiffness and Wear Resistance

Higher stiffness does not automatically imply a longer ski base service life.

The exceptional abrasive wear resistance of pure UHMWPE is related, among other factors, to its high toughness, elasticity and extremely long, entangled polymer chains. When exposed to an abrasive particle, an ice grain or another surface irregularity, the material can partially yield elastically without a fragment being immediately detached.

Adding a rigid reinforcing component limits deformation of the matrix and increases its dimensional stability. At the same time, however, local stresses may become concentrated around the particles.

If the amount, type or dispersion of the filler is unsuitable, the polymer matrix may be damaged around the particles, which can gradually loosen and be pulled out, causing the surface to crumble. Increased dimensional stability may therefore come at the cost of higher abrasive wear.

In simplified terms, pure UHMWPE absorbs part of the load through elastic deformation, whereas a stiffer composite material deforms less but may be more susceptible to local damage and particle separation.

There is therefore a trade-off between dimensional stability, machining quality, toughness and wear resistance.

The optimum ski base material need not be either the softest or the hardest. Its composition and internal structure must allow it to deform as little as possible during grinding, permit clean material separation, produce a minimum of unwanted fibres, retain the geometry of the structure under load, and at the same time preserve sufficient toughness and wear resistance.

The quality of the resulting structure is therefore determined not only by the grinding stone and machine settings. It is also significantly influenced by the design of the ski base material itself: the properties of the UHMWPE matrix, the type and quantity of added particles, their size and structure, the quality of their dispersion, and the course of the sintering process.

The same structure produced with the same grinding stone may therefore not have the same geometry, cleanliness or function on different materials. The ski base material is not merely a passive carrier of the structure. It actively determines how accurately the structure can be produced, how clean its surface will be, and to what extent it will retain its geometry under actual loading during skiing.

 

Effect of Ski Base Material on Structure Quality - Part No. 1

 Effect of Ski Base Material on Structure Quality

Ski base material affects the quality of a stone-ground structure primarily in two ways:

1.     through the formation of unwanted fibres and hairs during grinding,

2.     through the dimensional stability of the material during machining and subsequent loading.

A conventional racing ski base consists of an ultra-high-molecular-weight polyethylene (UHMWPE) matrix reinforced and modified with functional particles, most commonly carbon black, lamellar graphite and/or graphene.

How a Stone-Ground Structure Is Created

A stone-ground structure is most commonly produced using a rotating grinding stone whose surface has first been dressed to the required profile with a diamond dresser. The dresser therefore does not create the structure directly in the ski base; instead, it shapes the working surface of the grinding stone. When the stone subsequently comes into contact with the base, it transfers the prepared pattern into its surface by progressively removing material.

The resulting structure, however, is not a simple geometric imprint of the stone. It is produced by a dynamic machining process in which individual abrasive grains penetrate the UHMWPE surface, locally compress it, subject it to shear stress, deform it and ultimately separate material from it.

The actual shape of the grooves therefore depends not only on the profile of the grinding stone, but also on:

• the rotational speed of the stone,
• the ski feed speed,
• the applied pressure,
• the condition and grit of the stone,
• the dressing method,
• cooling and lubrication by the grinding emulsion,
• the mechanical properties of the ski base itself.

A soft and elastic material may partially yield in front of the abrasive grains and recover elastically after they have passed. The resulting groove may therefore be shallower, wider or less sharply defined than the geometric profile of the grinding stone would suggest.

A stiffer and more dimensionally stable material, by contrast, permits a more accurate transfer of the tool geometry. At the same time, however, it may be more susceptible to local matrix damage or particle pull-out.

Consequently, the same grinding programme and the same grinding stone may not produce an identical structure on different ski bases. The geometry of the grinding stone defines the intended shape, but the actual form of the structure is also determined by how the particular material responds to pressure, shear, elastic deformation and material separation.

The grinding stone therefore defines the structure geometrically, while the ski base material determines how accurately, cleanly and repeatably that geometry can be produced.

Formation of Unwanted Fibres

UHMWPE is a very tough material composed of exceptionally long polymer chains. During grinding, the material is not removed solely by clean cutting. Some polymer chains may be pulled out and stretched by an abrasive grain before they finally break. This produces unwanted fibres and hairs of varying lengths on the surface.

Their formation is also promoted by the internal heterogeneity of UHMWPE. The material contains alternating crystalline, amorphous and transitional regions with different strength, stiffness and machining behaviour. The length, density and distribution of the resulting fibres are therefore not entirely uniform.

Carbon black particles alter the internal structure of the polymer matrix and influence the way the material separates during grinding. With a suitable particle type, concentration and quality of dispersion, they can limit the extensive drawing-out of long polymer chains and promote shorter, cleaner separation of the machined material.

Carbon black may therefore create preferential sites for material separation. Damage does not necessarily propagate along a long section of a polymer chain, but may be locally interrupted in the region of a particle or particle aggregate. This limits the formation of long fibres protruding from the surface.

The result may be a less hairy surface, more precise groove edges and an overall cleaner cut. The quality of the result, however, is not determined solely by the amount of carbon black. Particle size, aggregate structure, uniformity of dispersion and the quality of bonding to the polymer matrix are equally important.

Dimensional Stability During Grinding

The second important parameter is material stiffness, expressed primarily by the modulus of elasticity, or E-modulus.

UHMWPE itself is tough but also relatively elastic. Under the action of an abrasive grain or the pressure exerted by the grinding stone, the surface therefore deforms elastically at first. Some of the material yields in front of the tool and, once the load is removed, returns at least partially to its original shape.

The more the material yields elastically during grinding, the more difficult it is to produce a geometrically precise structure. Groove edges may be less sharp, groove depth less uniform, and the final shape may deviate from the geometry of the grinding stone.

In this context, carbon black acts primarily as a reinforcing component of the polymer matrix. Its overall effect depends not only on the total amount added, but also on particle size and structure, aggregate size, the quality of dispersion within the polymer, bonding between the particles and UHMWPE, and the pressing and sintering conditions.

A well-designed compound can increase the material's modulus of elasticity, reduce deformation under load and thereby improve grinding accuracy. A stiffer ski base retains the shape of the grooves more effectively and allows the geometry of the grinding stone to be transferred more accurately to its surface.