Why Ski Base Matters – Part No. 2

What Actually Touches the Snow?

At the very beginning of sliding, assuming a freshly prepared and fully waxed ski base, the ski-base material itself may have little or no direct contact with the snow.

Instead, the initial contact takes place primarily between the snow surface and the additionally applied wax and coating layers. During the first several hundred metres, the outermost contact material may be a topcoat or a so-called speeder.

However, this initial condition does not remain unchanged.

Depending on snow conditions, mechanical wear and modification of the contacting surfaces can begin almost immediately.

Snow grains undergo elastic and plastic deformation, fracture, abrasion, reshaping and displacement. Some of these processes permanently modify the geometry and mechanical properties of the snow surface.

Importantly, these processes do not occur independently of the ski base. Every change in one part of the contacting system can influence the mechanical interaction with the other parts.

At the same time, wax and coating layers are exposed to local pressures, shear stresses and abrasive interactions. Their thickness, distribution and continuity can therefore change progressively during sliding.

Front and Rear Contact Windows – The Macroscopic Perspective

Depending on ski construction, camber profile, stiffness distribution and applied load, the contact between the ski and snow can be concentrated within two principal macroscopic regions: the front and rear contact windows.

The geometry, length, pressure distribution and relative loading of these contact windows depend on the particular ski and the actual skiing conditions.

Although both contact windows belong to the same ski, the physical processes taking place within them are not necessarily identical.

The front contact window interacts with the snow surface first and modifies the conditions subsequently encountered by the rear contact window.

As the front contact window passes over the snow, snow grains may undergo elastic and plastic deformation, fracture, abrasion, displacement and rearrangement.

These processes can modify the local snow-surface geometry, grain packing, contact stiffness and, under certain conditions, the distribution of interfacial water.

Consequently, the rear contact window encounters a snow surface that has already been mechanically and potentially thermally modified by the front contact window.

However, the snow surface does not necessarily remain unchanged between the passage of the two contact windows.

During this short interval, some elastic recovery, grain rearrangement and redistribution of interfacial water may occur.

The rear contact window therefore does not simply repeat the friction and wear processes occurring at the front.

Its real contact area, local pressure distribution, deformation mechanisms and friction regime may differ substantially.

The front and rear contact windows form two successive, mechanically coupled stages of the same sliding process.

This distinction is important when evaluating ski-base materials, structure patterns, waxes and coatings, because their performance may depend not only on the initial snow conditions but also on how those conditions have been modified by the preceding contact.

Distribution of Real Contact Points – The Microscopic Perspective

Within each macroscopic contact window, the actual contact between the ski and snow is not uniformly distributed across the structured ski-base surface.

The real contact occurs at a number of individual microscopic contact points, representing only a fraction of the nominal contact area.

Most of these contact points are expected to occur at the highest and most exposed features of the ski-base structure pattern.

However, depending on snow-grain size, shape, hardness, packing and deformation, some contact points may also develop inside the structure valleys.

Consequently, the distribution of real contact points is governed by the interaction between two geometrically and mechanically complex surfaces, rather than by the ski-base structure alone.

This distribution is also dynamic.

As snow grains deform, fracture, move or become rearranged, individual contact points can appear, disappear or shift to different locations.

At the same time, progressive wear of wax and coatings changes the materials exposed at these contact points.

The individual contact points also differ in size, local pressure and mechanical interaction. Some carry relatively small loads, while others experience much higher local stresses and more intensive abrasion.

These differences influence not only the deformation and wear of snow grains but also the removal of wax and coating materials from the ski-base surface.

Importantly, the microscopic contact conditions within the front and rear contact windows may be different, even when the structure pattern and surface preparation are identical.

The front contact window interacts with the original snow surface, whereas the rear contact window interacts with a surface that has already undergone a preceding mechanical interaction.

Therefore, the microscopic contact distribution, local pressures and wear mechanisms may differ between the two windows.

A Continuously Evolving Sliding Interface

The real ski–snow contact interface is not a fixed surface.

At the macroscopic level, the front and rear contact windows interact with the snow sequentially, under potentially different mechanical and thermal conditions.

At the microscopic level, individual contact points continuously change their geometry, location, local pressure and material composition.

Simultaneously, wax and coating layers are progressively modified or removed from the most exposed contact locations, while material retained in deeper structure valleys may remain protected for much longer.

The material actually touching the snow can therefore change during sliding—from the initially applied topcoat or speeder, through underlying wax layers, to the exposed ski-base material itself.

And this transition does not necessarily occur uniformly across the ski base or simultaneously within the front and rear contact windows.

The real sliding interface is a continuously evolving system in which snow properties, contact geometry, local pressure, surface materials and wear processes influence each other.

Understanding this evolution is essential when trying to explain why the properties of the ski-base material itself can become increasingly important during skiing.

Why Ski Base Matters – Part No. 1

If ski waxes and surface coatings worked exactly as they are often presented to work—that is, if they formed a continuous layer covering the entire ski-base surface, regardless of how complex the structure pattern is, and if this layer survived for several tens of kilometres—then the ski-base material itself should matter very little during a race.

Under such conditions, the ski base should hardly ever come into direct contact with the snow. The actual sliding interface would be formed primarily between the snow surface and the wax/coating layer.

In reality, however, the sliding interface and the associated wear processes are considerably more complex.

Modification of the Structure Pattern

The application of multiple wax layers and surface coatings modifies the original stone-ground structure.

The exact modification depends on the structure geometry, wax properties, application method, scraping and brushing procedure. In general, however, wax tends to make the structure shallower and its sharp features more rounded.

After scraping and brushing, wax is not distributed uniformly across the surface. A significant amount can remain accumulated in the deeper parts of the structure valleys, while the highest surface features are much more strongly exposed.

Consequently, the geometrical structure actually interacting with the snow can differ substantially from the freshly stone-ground structure.

Wear Process During Sliding on Snow

The real contact between ski and snow occurs only over a fraction of the nominal ski-base area.

Different friction regimes determine the number, size and total area of these real contact points, as well as the local pressures acting within them.

The complete interaction is complicated because both the snow and, to a much smaller extent, the ski-base surface can undergo elastic deformation, plastic deformation, fracture and wear. Nevertheless, several general relationships can be identified.

The smaller the real contact area and the fewer the individual contact points, the higher the local pressure within those contacts.

Higher local pressure becomes especially important when the snow is hard, transformed and compacted and when deformation deeper inside the snow cover is limited.

As a general tendency:

• fewer and smaller contact points produce higher local pressures;

• harder and more compact snow produces less deformation within the snow cover;

• reduced deformation concentrates the mechanical interaction closer to the ski/snow interface;

• higher local pressure and harder snow generally increase mechanical wear of the exposed ski-base surface.

Wear of Wax and Coatings

Wax and coating material is therefore not removed uniformly from the ski base.

Wear starts preferentially at the real contact points—normally the highest and most exposed irregularities of the structured base surface. These are the locations carrying the highest local loads and experiencing the strongest mechanical interaction with the snow.

At such points, the effective lifetime of a wax or coating layer can be very short.

The opposite situation exists deep inside the structure valleys.

Wax accumulated there is geometrically protected from direct mechanical contact with the snow and can therefore remain for a relatively long time. Its long persistence, however, does not necessarily demonstrate that it is providing an equally long-lasting low-friction sliding layer.

In fact, it may persist precisely because it is located outside the most important real contact zones.

This leads to an important distinction:

wax lifetime on the ski base is not the same thing as wax lifetime in the real ski–snow contact interface.

The highest structure features can lose their wax or coating very quickly, while material retained in deeper valleys remains almost untouched.

The ski-base material, its surface morphology and its mechanical and physicochemical properties can therefore become relevant to sliding performance much earlier than would be expected from the apparent persistence of wax on the ski.

Complex liquid gliders on ISANTIN coating

 We tested more complex liquid gliders called also as top coats or speeders on ISANTIN coating and we compared liquid glider’s gliding performance and stability if applied both on recommended wax base or ISANTIN coating.

 

We tested especially in wet, very wet and dirty abrasive man-made snow conditions. We tested at different speed rates and distances.

 

In the one set of test skis base layers were prepared as recommended by liquid glider manufacturer, in the other set of test skis base layer was built by ISANTIN coating.

 

Independently of the brand of complex liquid glider and used base layer the results for gliding performance and stability were always very similar.

 

Gliding performance on both wax base and ISANTIN base was very similar in first tests or runs, with increasing number of tests / runs the gliding performance of complex liquid gliders on wax base stayed or got worse only slightly but the gliding performance of complex liquid gliders on ISANTIN base got worse visibly.

 

Conclusions

 

Gliding performance of complex liquid gliders outperforms ISANTIN coating on wet to very wet dirty and abrasive man-made snow in the first several km.

 

Stability of gliding performance of complex liquid gliders is depending on base layer. Only if the base layer enables the complex liquid glider to be integrated into the base layer, stability is positively influenced. If the coating seals the ski base with a covering non-porous layer as ISANTIN does, the stability of gliding performance of complex liquid gliders is much smaller.

 

Open points

 

It would be very interesting to see how the gliding performance of both sets of skis changes after first km where skis with wax base do glide especially on wax layer and ISANTIN coating remains unchanged. 

 

 

 

 

 

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 1.11 The Boundaries Between the Individual Reservoirs Are Not Sharp

The three reservoirs described above form a useful working model, not three perfectly separated layers.

In reality, continuous transitions exist between them.

Wax filling a deeper groove in the ground structure is geometrically located above the polymer surface, but it may simultaneously be retained very strongly by mechanical interlocking.

At some locations, the thin surface film may be only adsorbed, while in a neighbouring amorphous region it may already partly penetrate beneath the geometric surface.

A microcrack or an interphase gap around an additive particle may simultaneously function as:

• a surface depression,
• a mechanical wax reservoir,
• an entry pathway into the polymer surface zone.

At different locations on the ski base, all three reservoirs may therefore coexist in different proportions.

1.12 Practical Importance of Dividing the Wax into Three Reservoirs

Dividing the wax into three reservoirs makes it possible to define the objective of scraping and brushing more precisely.

First reservoir

The macroscopic surplus should be removed almost completely. Leaving it in place would cover the structure and impair the function of the surface.

Second reservoir

The thin film should be reduced selectively. The ground structure must be opened and loose or excessively thick residues removed, but it is not self-evident that the optimum objective is to remove all wax from every mechanically accessible part of the surface.

Third reservoir

The wax within the polymer surface zone should remain preserved. Its removal is not part of the desired cleaning of the ski base, but rather a possible secondary consequence of abrasion of the UHMWPE itself.

The fundamental question in brushing is therefore not merely:

How quickly does the brush open the ground structure?

An equally important question is:

At what point does the brush stop removing technological surplus and begin reducing the functional film or the wax-enriched polymer surface zone itself?

This question will be examined in the following chapters dealing with manual and rotary steel brushes.

 

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 1.8 First Reservoir: The Macroscopic Wax Layer Above the Surface

The first and most clearly visible reservoir consists of wax that remains as a continuous layer above the original ski-base surface after ironing.

Its thickness may be locally relatively high, particularly where a large amount of wax has been applied or where the wax has not been distributed uniformly by the iron.

Wax filling the deeper parts of the ground structure also belongs to this category.

This is not wax absorbed within the polymer matrix. It is wax lying above the local UHMWPE surface and mechanically filling:

• grooves,
• depressions,
• grinding marks,
• open surface defects,
• spaces between protruding fibrils.

This layer is largely a technological surplus.

If it remained on the ski base, it would cover or significantly restrict the function of the ground structure. It could increase the real contact area, impair the removal of water and contaminants, and form a soft surface susceptible to the mechanical retention of dirt.

It is therefore intended to be removed primarily with a plastic scraper and by subsequent brushing.

The macroscopic reservoir is not equally thick everywhere. Its thickness varies along the X- and Y-axes according to the local topography, the amount of applied wax, the direction in which the iron was moved, and the geometry of the structure.

1.9 Second Reservoir: A Thin Wax Film Within the Surface Microstructure

After the macroscopic surplus has been removed, a very thin wax layer remains mechanically and adhesively bonded to the surface.

It is located in particular:

• on the peaks and walls of the ground structure,
• in fine scratches,
• between fibrils,
• in surface defects,
• in open microscopic depressions,
• at interfaces between differently oriented UHMWPE regions,
• around certain additive particles and agglomerates.

This second reservoir is no longer merely a thick technological surplus.

It may contribute significantly to the resulting:

• frictional properties,
• wettability,
• surface energy,
• resistance to contamination,
• initial glide performance.

It is, however, difficult to define precisely which part of this film is still excessive and which part is already functional.

Brushing must remove wax that blocks the ground structure, but it should not automatically mean removing all wax from the peaks, walls, and microroughness of the surface.

The second reservoir is the most vulnerable to excessive brushing because it is directly mechanically accessible to the tips and side surfaces of the brush filaments.

1.10 Third Reservoir: Wax Within the Uppermost Polymer Zone

The third reservoir consists of wax that has penetrated beneath the geometric surface into the uppermost part of the UHMWPE during heating.

This wax is not stored in ordinary open pores. It is located primarily:

• in surface-accessible amorphous regions,
• in partially accessible transitional regions,
• at interphase boundaries,
• in manufacturing and surface defects,
• in microcracks,
• in areas disturbed by grinding.

Its concentration is not uniform either across the surface or with depth.

Along the X- and Y-axes, regions capable of absorbing relatively more wax alternate with areas whose surfaces consist predominantly of crystalline material or additive particles and therefore have a very low absorption capacity.

Along the Z-axis, the wax concentration is likely to decrease rapidly from the surface into the material.

This reservoir is not normally accessible to the scraper or brush as free wax. A steel filament cannot simply “comb” it out of undamaged polymer.

It may, however, remove it indirectly if the filament begins mechanically removing the uppermost UHMWPE layer in which the wax is stored.

Even a very small loss of polymer may be significant in this respect if most of the absorbed wax is concentrated within the first few hundred nanometres of the surface.

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 1.6 Effect of Additives on the Ski Base’s Capacity to Absorb Wax

Various additives and solid lubricants are commonly incorporated into UHMWPE intended for ski bases, including carbon black, graphite, and other carbon-based materials. The use of carbon particles and solid lubricants in ski-base materials is well documented in the technical literature.

Additives may affect wax absorption through several mechanisms acting simultaneously.

1.6.1 Replacement of Part of the Polymer Volume

Additive particles occupy volume that is no longer formed by UHMWPE.

A particle of carbon black, graphite, or another filler cannot absorb wax through the same mechanism as the amorphous polymer phase. As the additive content increases, the volume of polymer matrix that is potentially capable of absorbing wax decreases.

From this basic volumetric perspective, the following tendency can be expected:

The higher the additive content, the lower the maximum volumetric capacity of the ski base to absorb wax into the UHMWPE.

1.6.2 Modification of Polymer Crystallisation

Additives may act as heterogeneous nucleation sites and promote the formation of crystals. At higher concentrations, however, they may simultaneously restrict the motion of macromolecules and inhibit the growth of crystalline regions.

Both of these opposing effects have been described for carbon fillers: heterogeneous nucleation as well as restricted crystal growth caused by reduced chain mobility.

The effect of an additive on wax absorption is therefore not simply a function of its weight fraction. It also depends on whether the particular additive:

• increases or decreases crystallinity,
• changes the size of the crystalline lamellae,
• creates a rigid interphase zone around the particles,
• disrupts the continuity of amorphous regions,
• creates new defects or gaps at the particle–polymer interface.

1.6.3 Interphase Region Around Particles

An interphase zone forms around an additive particle in which the arrangement of the polymer chains may differ from that in the unrestricted polymer matrix.

Depending on the quality of bonding between the particle and the UHMWPE, this region may be:

• denser and more rigid,
• less mobile,
• or, conversely, defective and contain microscopic gaps.

A well-bonded rigid interphase region may restrict wax penetration. A poorly bonded interface may locally create space for the mechanical retention or capillary entry of wax.

It cannot therefore be assumed that every additive and every interface always reduce wax absorption in the same manner.

1.6.4 Practical Overall Relationship

When otherwise comparable ski bases are considered, a high proportion of solid additives can generally be expected to reduce the total volume of polymer matrix capable of absorbing wax.

The practical rule “the more additives, the less wax” is therefore a physically reasonable starting assumption.

It must, however, be understood as a general tendency rather than an absolute linear law. The resulting wax-absorption capacity is also affected by:

• the type of additive,
• particle size,
• particle shape,
• degree of agglomeration,
• spatial distribution,
• quality of bonding to the UHMWPE,
• the additive’s influence on crystallisation,
• the formation of defects during sintering.

Microscopic analyses of actual ski-base materials show that carbon particles may be distributed relatively homogeneously within the matrix, while agglomerates and other particulate inclusions may also be present.

1.7 Depth of Wax Penetration into the Ski Base

The ski base cannot be understood as an openly porous material that absorbs wax to depths of tens or hundreds of micrometres during hot-wax application.

Wax molecules enter primarily the uppermost part of the polymer, which becomes sufficiently heated during application and contains accessible amorphous, transitional, or defective regions.

Penetration is restricted by several factors:

• the very short period during which the ski base is heated,
• the relatively low temperature compared with the melting point of UHMWPE,
• the high molecular weight and density of the polymer,
• the high proportion of crystalline phase,
• the limited diffusive mobility of wax molecules,
• the rapid decrease in temperature with depth,
• the lack of connection between some amorphous regions and the surface.

For a working technical model, it may be assumed that during normal hot-wax application, the great majority of the absorbed wax is concentrated within an extremely thin surface region.

The following may be considered as an approximate working range:

• the main proportion of absorbed wax within approximately the first 250 to 350 nm,
• occasional or significantly lower penetration to depths on the order of up to 1 μm.

These values must be understood as estimates of the characteristic depth of the surface zone, not as a sharp boundary.

There is no interface above which the polymer is uniformly saturated with wax and below which no wax is present. The wax concentration is likely to be highest immediately at the surface and to decrease rapidly with depth.

The actual penetration depth will depend in particular on:

• the molecular size and distribution of the wax components,
• the viscosity of the molten wax,
• the temperature of the ski base,
• the duration of heating,
• the number of repeated applications,
• the density and crystallinity of the UHMWPE,
• the interconnectivity of amorphous regions,
• the content and distribution of additives,
• the condition of the surface after grinding.

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 1.3 Which Regions Can Absorb Wax

Under normal conditions, wax molecules cannot freely penetrate perfectly ordered crystalline regions of UHMWPE.

In the crystalline phase, polymer chains are tightly packed and there is insufficient free volume between them for the easy diffusion of paraffin-wax molecules. Crystalline regions therefore represent effectively closed or very poorly accessible parts of the material with respect to wax penetration.

Wax can penetrate primarily into:

• amorphous regions,
• the less ordered parts of transitional regions,
• interphase boundaries,
• defects and voids created during sintering,
• microcracks and surface imperfections,
• regions disturbed by grinding or use.

The statement that amorphous and transitional regions can accept wax does not mean that all such regions are equally accessible.

Part of the amorphous phase may be enclosed between crystalline lamellae and may not be connected to the surface by a diffusion pathway. Transitional regions are also more structurally constrained than the fully amorphous phase. Their capacity to absorb wax may therefore be lower and depends on the local density and mobility of the polymer chains.

For practical wax impregnation of the ski base, the decisive factor is therefore not merely the total proportion of amorphous material, but primarily the proportion of amorphous and transitional regions that are:

• connected to the surface,
• sufficiently mobile at the application temperature,
• geometrically accessible to the molecules of the wax being used.

1.4 Relative Proportions of the Crystalline, Amorphous, and Transitional Phases

UHMWPE is a highly crystalline polymer, but the exact proportions of the individual phases are not universal.

They depend in particular on:

• the type and molecular weight of the original UHMWPE,
• the pressing and sintering conditions,
• the cooling rate,
• the presence of additives,
• the orientation of the material,
• subsequent thermal and mechanical processing.

In the conventional two-phase description, only crystalline and amorphous regions are usually distinguished. This model is, however, too simplified for describing wax absorption, because a significant proportion of the polymer chains may exhibit intermediate mobility between the perfectly crystalline and fully amorphous states.

For a working model of the ski base, it is therefore more appropriate to consider three groups:

• the crystalline phase,
• the transitional or rigid-amorphous phase,
• the more mobile amorphous phase.

It is also necessary to distinguish between:

• the volume fraction of the individual phases,
• the typical size of the individual domains,
• the total interfacial area between them.

A material may, for example, have a high proportion of crystalline phase while simultaneously possessing a very fine lamellar structure. In that case, a large total interfacial area may exist between the crystalline and amorphous regions.

Conversely, a material having the same overall degree of crystallinity but larger crystalline domains may have a smaller total area of transitional regions.

From the perspective of wax absorption, it is therefore not sufficient to know only the percentage crystallinity. The size and spatial arrangement of the crystalline lamellae, amorphous regions, and transitional interfaces are also important.

1.5 Relationship Between Domain Size and Wax Absorption

The size of the individual crystalline and amorphous regions may affect the ski base’s ability to absorb wax in several ways.

Larger crystalline regions

Larger and more perfectly ordered crystalline regions create more extensive areas that are difficult for wax to access. If the total interfacial area is simultaneously reduced, the number of pathways through which wax may enter the polymer surface zone may also decrease.

Smaller crystalline lamellae

At the same degree of crystallinity, a finer crystalline structure may create a larger total interfacial area between the crystalline and amorphous phases.

This may increase the amount of transitional material. It does not, however, automatically result in greater wax absorption, because some transitional regions may be rigid, dense, and poorly accessible to wax molecules.

Larger interconnected amorphous regions

More extensive and mutually interconnected amorphous regions may create easier diffusion pathways from the surface into the material.

Small isolated amorphous regions

An amorphous region enclosed between crystalline lamellae may be practically inaccessible to wax, even though it is included in the total proportion of the amorphous phase.

The decisive parameter is therefore not merely the amount of amorphous material, but also its:

• size,
• interconnectivity,
• distance from the surface,
• molecular mobility,
• connection to interphase and defective regions.